ar71xx: ubnt-(xm,xw): fix LED RSSI indication

ar71xx: ubnt-(xm,xw): fix LED RSSI indication

When mapping for RSSI LEDs was defined for interface wlan0 on
Ubiquiti XM and XW family, it missed connection to actual interface.
Therefore create the mapping to interface, so RSSI LEDs work without
additional configuration, after starting rssileds service.

Also add the required package for this.

While at that, remove coefficients needed for PWM LEDs, as XM and XW
boards do not support PWM LEDs.

Tested-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
[Squashed commits + remove custom device_packages + slighty rewrite the commit msg]
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>