HID: wacom: generic: Scale battery capacity measurements to percentages

The power_supply subsystem expects us to provide it with capacity values
measured in percent. In particular, AES devices (HID_DG_BATTERYSTRENGTH)
use the range 0-255, which needs to be rescaled. The MobileStudio Pro
(WACOM_HID_WD_BATTERY_LEVEL) uses the range 0-100, but there's no guarantee
that future devices will share the same range.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>