ALSA: usb-audio: scarlett_gen2: Fix -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.

Link: https://lore.kernel.org/r/20230915082802.28684-4-tiwai@suse.de


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