Input: wacom_w8001 - implement open and close

Implement open() and close() methods for the input device so that we
do not start the device unless there are users listening to the events.

Acked-by: Chris Bagwell <chris@cnpbagwell.com>
Tested-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>