mt76: move mt76x02_mac_write_txwi in mt76x02-lib module

Move mt76x02_mac_write_txwi in mt76x02_mac.c since it is shared between
mt76x0 and mt76x2 drivers. This is a preliminary patch to unify txwi
configuration between mt76x0 and mt76x2 drivers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>