HID: wacom: add full support of the Wacom Bamboo PAD

The stylus of this device works just fine out of the box.
The touch is seen by default as a mouse with relative events and some
gestures.
The wireless and the wired version have slightly different firmwares, but
the debug mode 2 on the feature 2 is common to the 2 devices. In this mode,
all the reports are emitted through the debug interface (pen, raw touch
and mouse emulation), so we have to re-route manually the events.

We keep the Pen interface as a HID_GENERIC one because it works, and only
parse the raw touches while discarding the mouse emulation & gestures.

Switching the default in raw mode allows us to have a consistent user
experience accross all the multitouch touchpads (and enable the touch part
of the devices).

Note that the buttons of this devices are reported through the touch
interface. There is no 'Pad' interface. It seemed more natural to have
the BTN_LEFT and BTN_RIGHT reported with the touch because they are
placed under the touch interface and it looks like they belong to the
touch part.

Tested-by: Josep Sanchez Ferreres <josep.sanchez.ferreres@est.fib.upc.edu>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>