power: supply: ab8500_charger: Fix VBAT interval check

When using USB charging, the AB8500 charger is periodically
checking VBAT for a threshold at 3.8V.

This crashes badly, as the class_for_each_device() was passed
the wrong argument. I think this has maybe worked by chance
in the past because of how the structs were arranged but it
is leading to crashes now.

Fix this up and also switch to using microvolts for the
voltages like the rest of the code.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>