mt76: move mt76x02_phy_set_band in mt76x02-lib module

Move mt76x02_phy_set_band routine in mt76x02_phy.c since
it is shared between mt76x0 and mt76x2 drivers and remove
duplicated code

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