ALSA: pcxhr: Fix missing mutex unlock

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: 9bef72bdb2 ('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>

sound/pci/pcxhr/pcxhr_core.c

@@ -1341,5 +1341,6 @@ irqreturn_t pcxhr_threaded_irq(int irq, void *dev_id)
 	}
 
 	pcxhr_msg_thread(mgr);
+	mutex_unlock(&mgr->lock);
 	return IRQ_HANDLED;
 }