HID: wacom: generic: Report AES battery information

When support for the HID_DG_BATTERYSTRENGTH usage was added for AES devices,
it appears that the value was read, but never actually forwarded to the
power_supply subystem for userspace's benefit. Let's correct that.

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>