ALSA: firewire-motu: fix double unlocked 'motu->mutex'

Mutex is doubly unlocked in some error path of pcm.open. This commit fixes
ALSA firewire-motu driver in Linux kernel v5.5.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3fd80b20 ("ALSA: firewire-motu: use the same size of period for PCM substream in AMDTP streams")
Fixes: 0f5482e7

 ("ALSA: firewire-motu: share PCM buffer size for both direction")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191208232226.6685-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>