mt76: mt7615: rearrange locking in mt7615_config

Since all the routines in mt7615_config grub mt76.mutex moves
mutex_lock/mutex_unlock at the beginning/end of mt7615_config

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