HID: wacom: remove the extra Pen interface for Wacom Bamboo PAD

As mentioned in the comment in the code, both the pen and touch data
come from the interface tagged as BAMBOO_PAD. The driver re-routes the
events for the Pen to the generic HID interface and keeps the ones for
the touch through this current interface.

Clearing the WACOM_DEVICETYPE_PEN bit removes the extra unused interface
added in 2a6cdbdd ("HID: wacom: Introduce new 'touch_input' device") and
makes the Bamboo PAD to behave like in 4.1.

Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>