ALSA: snd-usb-caiaq: fix stream count check

Commit 897c329b ("ALSA: usb: caiaq: check for cdev->n_streams > 1")
introduced a safety check to protect against bogus data provided by
devices. However, the n_streams variable is already divided by
CHANNELS_PER_STREAM, so the correct check is 'n_streams > 0'.

Fix this to un-break support for stereo devices.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Cc: stable@kernel.org [v3.18+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>