ALSA: pcxhr: Fix missing mutex unlock

ALSA: pcxhr: Fix missing mutex unlock

commit 67f3754b51f22b18c4820fb84062f658c30e8644 upstream.

The commit [9bef72bdb26e: ALSA: pcxhr: Use nonatomic PCM ops]
converted to non-atomic PCM ops, but shamelessly with an unbalanced
mutex locking, which leads to the hangup easily.  Fix it.

Fixes: 9bef72bdb26e ('ALSA: pcxhr: Use nonatomic PCM ops')
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=116441
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>