Battery device names must be unique, otherwise registration fails if
multiple Wii Remotes are connected.

This breaks the sysfs API, but there is no known application that uses the
Wii Remote battery that I know of so we should go ahead and apply this.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>