USB: input: xpad.c: fix up dev_err() usage

We should always reference the input device for dev_err(), not the USB
device.  Fix up the places where I got this wrong.

Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
CC: "Magnus Hörlin" <magnus@alefors.se>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>