ar71xx: ubnt-rocket-m-ti: fix RSSI LED definitions

ar71xx: ubnt-rocket-m-ti: fix RSSI LED definitions

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

While at that, split RSSI into ~equal intervals for 6 LEDs,
and remove coefficients needed for PWM LEDs, as this board does not
support PWM LEDs.

Finally, for complete support, enable 'rssileds' package in per-device
rootfs, so the indicator works out of box.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>