mt76: move mt76x02_get_txpower in mt76x02_util.c

Move mt76x02_get_txpower utility routine in mt76x02-lib module
in order to be reused by mt76x0 and mt76x2u drivers

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