mt76x2: move mt76x2_phy_tssi_compensate in mt76x2-common module

Move mt76x2_phy_tssi_compensate routine in mt76x2-common module
since it is shared between mt76x2 and mt76x2u drivers

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