[ALSA] usbaudio - Fix Oops with broken usb descriptors

This is a patch for ALSA Bug #2724. Some webcams provide bogus
settings with no valid rates. With this patch those are skipped.

Signed-off-by: Gregor Jasny <gjasny@web.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>