power: supply: ab8500_fg: remove comparison to bool

Fix the following coccicheck warning:

drivers/power/supply/ab8500_fg.c:2402:5-24: WARNING: Comparison to bool

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