HID: wacom: Fix invalid power_supply_powers calls

power_supply_powers calls added in 35b4c01e ("power_supply: add "powers" links
to self-powered HID devices") have to be called after power device is created.
This patch also fixes the second call - it has to be "ac" instead of "battery"

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