power_supply: ab8500_fg: Simplify creation and removal of sysfs entries

Simplify a little ab8500_fg_sysfs_psy_create_attrs () and
ab8500_fg_sysfs_psy_remove_attrs() functions because they received
pointer to power supply device which was then converted into power
supply instance. Then it was converted into struct ab8500_fg. The path
looked like:
	ab8500_fg->psy.dev -> psy -> ab8500_fg

Instead just pass pointer to struct ab8500_fg directly so all
conversions won't be necessary.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>