ALSA: mixart: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in this particular case, I replaced the code comment with
a proper "fall through" annotation, which is what GCC is expecting
to find.

Addresses-Coverity-ID: 114889 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart_core.c

@@ -540,7 +540,7 @@ irqreturn_t snd_mixart_threaded_irq(int irq, void *dev_id)
 				dev_err(&mgr->pci->dev,
 					"canceled notification %x !\n", msg);
 			}
-			/* no break, continue ! */
+			/* fall through */
 		case MSG_TYPE_ANSWER:
 			/* answer or notification to a message we are waiting for*/
 			mutex_lock(&mgr->msg_lock);