USB: input: wacom_sys.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: Ping Cheng <pingc@wacom.com>
CC: Chris Bagwell <chris@cnpbagwell.com>
CC: Eduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>