power: supply: ab8500_fg: Allocate wq in probe

The workqueue is allocated in bind() but all interrupts are
registered in probe().

Some interrupts put work on the workqueue, which can have
bad side effects.

Allocate the workqueue in probe() instead, destroy it in
.remove() and make unbind() simply flush the workqueue.

Fixes: 1c1f13a0 ("power: supply: ab8500: Move to componentized binding")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>