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/20230915082802.28684-10-tiwai@suse.de


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