ALSA: mixart: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart.c

@@ -182,6 +182,7 @@ static int mixart_set_clock(struct mixart_mgr *mgr,
 	case PIPE_RUNNING:
 		if(rate != 0)
 			break;
+		/* fall through */
 	default:
 		if(rate == 0)
 			return 0; /* nothing to do */