ALSA: pcxhr: Fix missing mutex unlock

The commit [9bef72bd: 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: 9bef72bd ('ALSA: pcxhr: Use nonatomic PCM ops')
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=116441


Cc: <stable@vger.kernel.org> # 3.18+
Signed-off-by: Takashi Iwai <tiwai@suse.de>