mt76: add missing lock configuring coverage class

mt76: add missing lock configuring coverage class

Coverage class callback can potentially run in parallel with other
routines (e.g. mt7615_set_channel) that configures timing registers.
Run coverage class callback holding mt76 mutex

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