HID: wacom: PAD is independent with pen/touch

PAD can be on pen interface (Intuos Pro and Cintiq series) or touch
interface (Bamboo PT and Intuos PT series) or its own interface
(Bamboo pen-only and Intuos Pen M/S). We need to mark it independently.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>