mt76x0: phy: use proper name convention

Use mt76x0_phy as prefix for routines in mt76x0/phy.c
Moreover use mt76x0_rf_set to enable vco calibration in
mt76x0_phy_vco_cal

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