Input: wacom - add support for Bamboo Pen

This adds support for Pen on Bamboo Pen and Bamboo Pen&Touch devices.
Touchpad is handled by previous Bamboo Touch logic.

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