power_supply: apm_power: Use power_supply_*() API for accessing function attrs

Replace direct calls to power supply function attributes with wrappers.
Wrappers provide safe access in case of unregistering the power
supply (e.g. by removing the driver). Replace:
 - get_property -> power_supply_get_property

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>