usb: usbip: Avoid NULL pointer dereference in case of error

One line above we have checked that udc is NULL so we shouldn't
dereference it while printing error message.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>