mt76: move mt76x02_tx_get_max_txpwr_adj in mt76x02_util.c

Move mt76x02_tx_get_max_txpwr_adj routine in mt76x02-lib module
since now both mt76x0 and mt76x2 drivers read rate tx power gain from
rate_power data structure. Moreover remove get_max_txpwr_adj function
pointer from mt76_driver_ops data structure

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