power: supply: ab8500: Avoid NULL pointers

Sometimes the code will crash because we haven't enabled
AC or USB charging and thus not created the corresponding
psy device. Fix it by checking that it is there before
notifying.

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