HID: wacom: Provide battery charge state to system over USB if available

If a wireless adapter (which contains the charging circuitry) is
detected as being attached to the tablet then create a new battery
interface and update its status as data is reported. Also destroy the
battery if the adapter goes away.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>