ALSA: hda - Add a fallthru comment

... to improve the readability.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/hda_beep.c

@@ -110,6 +110,7 @@ static int snd_hda_beep_event(struct input_dev *dev, unsigned int type,
 	case SND_BELL:
 		if (hz)
 			hz = 1000;
+		/* fallthru */
 	case SND_TONE:
 		if (beep->linear_tone)
 			beep->tone = beep_linear_tone(beep, hz);