mt76: mt7615: release mutex in mt7615_reset_test_set

Reduce scope of mutex_acquire/mutex_release in mt7615_reset_test_set
routine in order to fix the following static checker warning:

drivers/net/wireless/mediatek/mt76/mt7615/debugfs.c:179
mt7615_reset_test_set()
warn: inconsistent returns 'dev->mt76.mutex'.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: ea4906c4 ("mt76: mt7615: wake device before accessing regmap in debugfs")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>