ALSA: caiaq: Fix stray URB at probe error path

caiaq driver doesn't kill the URB properly at its error path during
the probe, which may lead to a use-after-free error later.  This patch
addresses it.

Reported-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>