ab8500_fg: Harden platform data check

If no platform data at all is supplied the driver crashes,
extend the checks to be more careful so we can compile in the
driver and boot also without platform data present.

Acked-by: Arun Murthy <arun.murthy@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>