HID: wacom: generic: Send BTN_STYLUS3 when both barrel switches are set

The Wacom Pro Pen 3D includes a third barrel switch which is intended to
be particularly useful in applications where one frequency uses pan, zoom,
and rotate to navigate around a scene or model. The pen is compatible with
the MobileStudio Pro, 2nd-gen Intuos Pro, and Cintiq Pro. When the third
button is pressed, these devices set both the HID_DG_BARRELSWITCH and
HID_DG_BARRELSWITCH2 usages since their HID descriptors do not include a
usage specific to the button.

Rather than send both BTN_STYLUS and BTN_STYLUS2 when the third button is
pressed, userspace (libinput) has requested that we detect this condition
and report a newly-defined BTN_STYLUS3 event instead. We could define a
quirk specific to devices compatible with the Pro Pen 3D, but the liklihood
of seeing both barrel switch bits set with other pens/devices is low enough
to not worry about (pens mechanically prevent accidental activation of
multiple switches).

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>