ab8500_fg: Reorder obtainment of platform specific battery management data

Now that we always pass platform specific battery management data
through platform_data instead of obtaining it via different means
depending the way be boot the system (DT or ATAGs); we need to
re-jiggle the way we acquire it in the driver start-up functions.
Now it is wrong for it to be missing, but we still allow Device
Tree code to fiddle with it once we've confirmed it's there.

Signed-off-by: Lee Jones <lee.jones@linaro.org>