iwinfo: show txpower from 0 to 31 dbm in txpowerlist

root@iopsys-44D43771AD60:~# ubus call iwinfo txpowerlist '{"device":"wl0"}'
{
        "results": [
                {
                        "dbm": 31,
                        "mw": 1258,
                        "active": false
                },
                {
                        "dbm": 30,
                        "mw": 1000,
                        "active": false
                },
                {
                        "dbm": 29,
                        "mw": 794,
                        "active": true
                },
                {
                        "dbm": 28,
                        "mw": 630,
                        "active": false
                },
                {
                        "dbm": 27,
                        "mw": 501,
                        "active": false
                },
                {
                        "dbm": 26,
                        "mw": 398,
                        "active": false
                },
                {
                        "dbm": 25,
                        "mw": 316,
                        "active": false
                },
                {
                        "dbm": 24,
                        "mw": 251,
                        "active": false
                },
                {
                        "dbm": 23,
                        "mw": 199,
                        "active": false
                },
                {
                        "dbm": 22,
                        "mw": 158,
                        "active": false
                },
                {
                        "dbm": 21,
                        "mw": 125,
                        "active": false
                },
                {
                        "dbm": 20,
                        "mw": 100,
                        "active": false
                },
                {
                        "dbm": 19,
                        "mw": 79,
                        "active": false
                },
                {
                        "dbm": 18,
                        "mw": 63,
                        "active": false
                },
                {
                        "dbm": 17,
                        "mw": 50,
                        "active": false
                },
                {
                        "dbm": 16,
                        "mw": 39,
                        "active": false
                },
                {
                        "dbm": 15,
                        "mw": 31,
                        "active": false
                },
                {
                        "dbm": 14,
                        "mw": 25,
                        "active": false
                },
                {
                        "dbm": 13,
                        "mw": 19,
                        "active": false
                },
                {
                        "dbm": 12,
                        "mw": 15,
                        "active": false
                },
                {
                        "dbm": 11,
                        "mw": 12,
                        "active": false
                },
                {
                        "dbm": 10,
                        "mw": 10,
                        "active": false
                },
                {
                        "dbm": 9,
                        "mw": 7,
                        "active": false
                },
                {
                        "dbm": 8,
                        "mw": 6,
                        "active": false
                },
                {
                        "dbm": 7,
                        "mw": 5,
                        "active": false
                },
                {
                        "dbm": 6,
                        "mw": 3,
                        "active": false
                },
                {
                        "dbm": 5,
                        "mw": 3,
                        "active": false
                },
                {
                        "dbm": 4,
                        "mw": 2,
                        "active": false
                },
                {
                        "dbm": 3,
                        "mw": 1,
                        "active": false
                },
                {
                        "dbm": 2,
                        "mw": 1,
                        "active": false
                },
                {
                        "dbm": 1,
                        "mw": 1,
                        "active": false
                },
                {
                        "dbm": 0,
                        "mw": 1,
                        "active": false
                }
        ]
}