mt76: mt7603: reset STA_CCA counter setting the channel

Reset MT_MIB_STAT_CCA after channel switch since it is used to track busy
time starting from 'commit dcff8d4d ("mt76: mt7603: switch to a
different counter for survey busy time")'

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>