HID: wacom: generic: Add support for vendor-defined "Fingerwheel" usage

The airbrush fingerwheel does not have a usage that corresponds cleanly
with a standard HID usage, so we add explicit support for it via its
vendor-defined usage.

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>