Move mt76_get_antenna in mac80211.c in order to be reused by all
drivers. Initialize .get_antenna function pointer for mt76x0, mt7603,
mt7615 and mt76x2u drivers
Signed-off-by: NLorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: NFelix Fietkau <nbd@nbd.name>