USB: input: wacom_sys.c: fix up dev_* messages

USB: input: wacom_sys.c: fix up dev_* messages

Previously I had made the struct device point to the input device, but
after talking with Dmitry, he said that the USB device would make more
sense for this driver to point to.  So converted it to use that instead.

CC: 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>