Backport: wacom - Add support for Intuos Pen Medium

Signed-off-by: Ping Cheng <pingc@wacom.com>
Backport: 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.

3.17 is not updated since the newly switched driver under HID-
subsystem isn't quite stable yet. We'll synchronize it with
upstream when 3.18-stable is available.

Signed-off-by: Ping Cheng <pingc@wacom.com>