Input: xpad - return proper error in error path

In current implementation, xpad_probe return 0 when
usb_alloc_urb failed for xpad->bulk_out and kzalloc failed for xpad->bdata.

This patch removes the initialization for error variable,
assign the error code at the place the error happens instead.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>