HID: wacom: Refactor battery/ac reporting for Graphire

This patch doesn't change the way battery/ac is reported, but the changes are
required to facilitate battery reporting for Intuos4 WL.
wdata->battery_capacity now stores actual battery capacity as opposed to raw
value reported by wacom graphire previously. Power supply state is now stored
in a separate variable - it used to be calculated on-the-fly in
wacom_ac_get_property function. The raw value has to be stored as well to be
able to determine if it has changed.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>