HID: wacom: generic: Extend pad support

The HID specification that the MobileStudio Pro follows includes usages
for several values that would be good to support so that future devices
"just work" out of the box. Extend the HID_GENERIC pad codepath to handle
these usages.

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