ISDN: Add check for usb_alloc_urb() result

usb_alloc_urb() return value needs to be checked to avoid
later NULL pointer access.
Reported by rucsoftsec@gmail.com via bugzilla.kernel.org #44601.

Signed-off-by: Karsten Keil <kkeil@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>