HID: wacom: Add battery/ac reporting for Intuos4 WL

This patch adds battery/ac reporting for Intuos4 WL. It uses existing
sysfs code, but the device reports battery capacity in more fine-grained way,
so there has to be a separate lookup table (called batcap_i4).

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