mt76: move mt76_get_rate in mt76-module

Move mt7603_get_rate in mac80211.c and rename it to mt76_get_rate
since it is shared between mt7603 and mt7615 drivers

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