ALSA: opl3: Mark expected switch fall-through

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

Addresses-Coverity-ID: 114878 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/opl3/opl3_midi.c

@@ -368,6 +368,7 @@ void snd_opl3_note_on(void *p, int note, int vel, struct snd_midi_channel *chan)
 			instr_4op = 1;
 			break;
 		}
+		/* fall through */
 	default:
 		spin_unlock_irqrestore(&opl3->voice_lock, flags);
 		return;