power: supply: ab8500: Rename charging algorithm psy

If we rename the "abx500_chargalg" supply to "ab8500_chargalg"
as it should be named, the existing supplies are supplying that
supply but that was obviously not working since it had the
wrong name.

Now that the dependency kicks in we get a bunch
of NULL references from ab8500_chargalg_external_power_changed()
so check that the workqueue is allocated before we try to
queue work on it.

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