ALSA: usb: caiaq: check for cdev->n_streams > 1

Coverity spotted a possible DIV0 condition when cdev->n_streams is 0.

Fix this by making sure the value is > 1 in snd_usb_caiaq_audio_init().

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>