power: supply: ab8500: Fix memory leak in ab8500_fg_sysfs_init

kobject_init_and_add() takes reference even when it fails.
According to the doc of kobject_init_and_add()：

   If this function returns an error, kobject_put() must be called to
   properly clean up the memory associated with the object.

Fix memory leak by calling kobject_put().

Fixes: 8c0984e5 ("power: move power supply drivers to power/supply")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>