ALSA: pcm: Fix snd_pcm_format_name function

Fix snd_pcm_format_name so it won't return NULL-pointer in case if it
can't find the format in the 'snd_pcm_format_names' list. Return
"Unknown" instead, as it is done if the number passed to the function
is larger than a list size.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240125223522.1122765-2-ivan.orlov0322@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>