mt76: generalize mt76_get_txpower for 4x4:4 devices

Genralize mt76_get_txpower routine for 4x4:4 capable devices
in order to be reused in mt7615 driver

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>