ALSA: hda - Add a fallthru comment

... to improve the readability.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/hda/hda_beep.c b/sound/pci/hda/hda_beep.c
index 20d065a1..98bce98 100644
--- a/sound/pci/hda/hda_beep.c
+++ b/sound/pci/hda/hda_beep.c
@@ -110,6 +110,7 @@
 	case SND_BELL:
 		if (hz)
 			hz = 1000;
+		/* fallthru */
 	case SND_TONE:
 		if (beep->linear_tone)
 			beep->tone = beep_linear_tone(beep, hz);