power: supply: ab8500: Avoid NULL pointers

[ Upstream commit 5bcb5087c9dd3dca1ff0ebd8002c5313c9332b56 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>