ALSA: usb-audio: unlock on error in probe

We need to unlock before we returning on this error path.

Fixes: 73ac9f5e

 ("ALSA: usb-audio: Add boot quirk for MOTU M Series")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200115174604.rhanfgy4j3uc65cx@kili.mountain
Signed-off-by: Takashi Iwai <tiwai@suse.de>