HID: wiimote: Read wiimote battery charge level

This registers a power_supply device for every remote to retrieve the current
battery charge level. Since this information is not sent by the wiimote
continously, we need to explicitely request it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>