libwifi: mtk: add iface_sta_info API. No info yet.

sta info call returns 0. wifi_sta is not filled with proper data yet.
Following info is provided by upper layer.
ubus call wifi.backhaul.wlan1_1 status
{
"ifname": "wlan1_1",
"status": "up"
}

Signed-off-by: Bartlomiej Grzeskowiak bartlomiej.grzeskowiak@iopsys.eu

@janusz @Marek please review