isdn: hisax: add missing usb_free_urb

Add missing usb_free_urb() on failure path in st5481_setup_usb().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Marina Makienko <makienko@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>