ALSA: usb-audio: Always initialize fixed_rate in snd_usb_find_implicit_fb_sync_format()

Handle the fallback code path, too.

Fixes: fd28941c

 ("ALSA: usb-audio: Add new quirk FIXED_RATE for JBL Quantum810 Wireless")
BugLink: https://lore.kernel.org/alsa-devel/Y7frf3N%2FxzvESEsN@kili/
Reported-by: Dan Carpenter <error27@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230109141133.335543-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>