ALSA: xen-front: fix unsigned error check on return from to_sndif_format

The negative error return from the call to to_sndif_format is being
assigned to an unsigned 8 bit integer and hence the check for a negative
value is always going to be false.  Fix this by using ret as the error
return and hence the negative error can be detected and assign
the u8 sndif_format to ret if there is no error.

Detected by CoverityScan, CID#1469385 ("Unsigned compared against 0")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamoccchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/xen/xen_snd_front_alsa.c

@@ -466,13 +466,14 @@ static int alsa_prepare(struct snd_pcm_substream *substream)
 		u8 sndif_format;
 		int ret;
 
-		sndif_format = to_sndif_format(runtime->format);
-		if (sndif_format < 0) {
+		ret = to_sndif_format(runtime->format);
+		if (ret < 0) {
 			dev_err(&stream->front_info->xb_dev->dev,
 				"Unsupported sample format: %d\n",
 				runtime->format);
-			return sndif_format;
+			return ret;
 		}
+		sndif_format = ret;
 
 		ret = xen_snd_front_stream_prepare(&stream->evt_pair->req,
 						   &stream->sh_buf,