iwinfo: fix txpowerlist calculation

calculation is done in mw power & then convert it to dbm.

root@iopsys:~# ubus call iwinfo txpowerlist '{"device":"wl1"}'
{
	"results": [
		{
			"dbm": 31,
			"mw": 1258,
			"active": true
		},
		{
			"dbm": 30,
			"mw": 1000,
			"active": false
		},
		{
			"dbm": 28,
			"mw": 630,
			"active": false
		},
		{
			"dbm": 25,
			"mw": 316,
			"active": false
		},
		{
			"dbm": 21,
			"mw": 125,
			"active": false
		}
	]
}