ALSA: ump: Shut up truncated string warning

The recent change for the legacy substream name update brought a
compile warning for some compilers due to the nature of snprintf().
Use scnprintf() to shut up the warning since the truncation is
intentional.

Fixes: e29e504e ("ALSA: ump: Indicate the inactive group in legacy substream names")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411300103.FrGuTAYp-lkp@intel.com/
Link: https://patch.msgid.link/20241130090009.19849-1-tiwai@suse.de


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