ALSA: mixart: Fix mutex deadlock

The code change for switching to non-atomic mode brought the
unexpected mutex deadlock in get_msg().  It converted the spinlock
with the existing mutex, but there were calls with the already holding
the mutex.  Since the only place that needs the extra lock is the code
path from snd_mixart_send_msg(), remove the mutex lock in get_msg()
and apply in the caller side for fixing the mutex deadlock.

Fixes: 8d3a8b5c ("ALSA: mixart: Use nonatomic PCM ops")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201119121440.18945-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>