HID: wacom: generic: Introduce pad support

As with usages for the pen, the Custom HID specificiation includes
usages for the pad. Here we add functions to map and handle most
of the pad usages present on the MobileStudio Pro.

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>