Manufacturers do not always populate serial number in their devices, so
let's fall back to device ID when forming the battery device name. As a
result, batteries in devices without serial number will be named like
this:

	hid-0018:2D1F:510E.0001-battery

(as opposed to hid--battery for the first one, and failing to create
batteries for the subsequent ones).
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>