ALSA: opti9x: Fix -Wformat-truncation warning

The filling of card->longname can be gracefully truncated, as it's
only informative.  Use scnprintf() and suppress the superfluous
compile warning with -Wformat-truncation.

Link: https://lore.kernel.org/r/20230915082802.28684-9-tiwai@suse.de


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