ALSA: xen: Fix -Wformat-truncation warning

The compile warning with -Wformat-truncation at
xen_snd_front_cfg_card() is false-positive; the loop can be only for
SNDRV_PCM_DEVICES which is at most 32.

For suppressing the warning, replace snprintf() with scnprintf().
As stated in the above, truncation doesn't matter.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/xen/xen_snd_front_cfg.c

···
 	*stream_cnt = 0;
 	num_devices = 0;
 	do {
-		snprintf(node, sizeof(node), "%d", num_devices);
+		scnprintf(node, sizeof(node), "%d", num_devices);
 		if (!xenbus_exists(XBT_NIL, xb_dev->nodename, node))
 			break;