Input: wacom - implement suspend and autosuspend

This implements suspend and autosuspend support for wacom devices.
It works by using the usb last busy functionality triggered in the
completion callback.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>