HID: logitech-hidpp: battery: provide CAPACITY_LEVEL

CAPACITY LEVEL allows to forward rough information on the battery mileage.
HID++ 2.0 devices will either report percentage or levels, so better
forwarding this information to the user space.

The M325 supports only 2 levels: 'Full' and 'Critical'. With mileage,
it will report either 90% or 5%, which might confuse users. With this
change the battery will either report "Full" or "Critical".

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