ALSA: pcm: Mark expected switch fall-through

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

Addresses-Coverity-ID: 1357375 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/pcm_plugin.c

@@ -353,6 +353,7 @@ snd_pcm_format_t snd_pcm_plug_slave_format(snd_pcm_format_t format,
 				if (snd_mask_test(format_mask, (__force int)format1))
 					return format1;
 			}
+			/* fall through */
 		default:
 			return (__force snd_pcm_format_t)-EINVAL;
 		}