mt76: add get_tx_txpwr_adj function pointer to mt76_driver_ops

Add get_tx_txpwr_adj function pointer to mt76_driver_ops data structure
as a preliminary patch to unify txwi configuration between mt76x0 and
mt76x2 drivers since tpc is currently supported just by mt76x2 driver

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