ALSA: snd-usb: avoid dividing by zero on invalid input

Signed-off-by: Nicolai Krakowiak <nicolai.krakowiak@gmail.com>
Acked-by: Daniel Mack <zonque@gmail.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>