HID: wacom - Bamboo pen-only tablet does not support PAD

Bamboo models do not support HID_DG_CONTACTMAX. Plus, Bamboo pen-only
has touch descriptor. This leads to some complications in the code.

This patch also fixes duplicated PAD interfeaces for Intuos Pen
models.

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