power: supply: ab8500_fg: Break out load compensated voltage

Break out the part of the function providing the load compensated
capacity that provides the load compensated voltage and use
that to get the load compensated capacity.

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