mt76: move tpc routines in mt76x02-lib module

Move mt76x02_tx_get_txpwr_adj and mt76x02_tx_set_txpwr_auto routines
in mt76x02-lib module since they are shared between mt76x0 and mt76x2
drivers. Moreover remove get_txpwr_adj function pointer

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