mt76x0: phy: do not run calibration during channel switch

Do not perform phy/vga calibration during channel switch.
Moreover remove mt76x0_agc_save and mt76x0_agc_restore routines
since they are no longer necessary. Furthermore run mt76_set_channel
in order to check if there are pending frames during channel switch

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