ALSA: pcm: Add fallthru comments

Just to improve readability.
Spotted by coverity CID 115002 and 115003.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_native.c

@@ -2428,6 +2428,7 @@ static int snd_pcm_hwsync(struct snd_pcm_substream *substream)
 	case SNDRV_PCM_STATE_DRAINING:
 		if (substream->stream == SNDRV_PCM_STREAM_CAPTURE)
 			goto __badfd;
+		/* Fall through */
 	case SNDRV_PCM_STATE_RUNNING:
 		if ((err = snd_pcm_update_hw_ptr(substream)) < 0)
 			break;
@@ -2460,6 +2461,7 @@ static int snd_pcm_delay(struct snd_pcm_substream *substream,
 	case SNDRV_PCM_STATE_DRAINING:
 		if (substream->stream == SNDRV_PCM_STREAM_CAPTURE)
 			goto __badfd;
+		/* Fall through */
 	case SNDRV_PCM_STATE_RUNNING:
 		if ((err = snd_pcm_update_hw_ptr(substream)) < 0)
 			break;