ALSA: usb-audio: scarlett_gen2: Fix another -Wformat-truncation warning

ALSA: usb-audio: scarlett_gen2: Fix another -Wformat-truncation warning

The recent enablement of -Wformat-truncation leads to a false-positive
warning for mixer_scarlett_gen2.c.

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

Fixes: 78bd8f5126f8 ("ALSA: usb-audio: scarlett_gen2: Fix -Wformat-truncation warning")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20230919071205.10684-1-peter.ujfalusi@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>