power: supply: ab8500_fg: Break loop for measurement

In the Samsung code tree we find that it can happen that this
measurement loop goes on for a long time, and it seems like a
good idea to break it after 70 iterations if it goes on for
too long.

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