HID: logitech-hidpp: rename battery level into capacity

The power_supply term for the percentage is capacity. Capacity level
can be given when non accurate mileage is provided by the device, so
better stick to the terms used in power_supply.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>