ALSA: scarlett2: Fix yet more -Wformat-truncation warnings

The recent code change introduced a few false-positive compile
warnings with -Wformat-trucation again.  Suppress them by replacing
snprintf() with scnprintf().

Fixes: 0a995e38 ("ALSA: scarlett2: Add support for software-controllable input gain")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401062344.AzZCYlpa-lkp@intel.com/
Link: https://lore.kernel.org/r/20240112171000.31855-1-tiwai@suse.de


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