power: supply: ds2780: Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
drivers/power/supply/ds2780_battery.c:627:60-61: WARNING opportunity for
kobj_to_dev()
drivers/power/supply/ds2780_battery.c:672:60-61: WARNING opportunity for
kobj_to_dev()
drivers/power/supply/ds2780_battery.c:640:60-61: WARNING opportunity for
kobj_to_dev()
drivers/power/supply/ds2780_battery.c:685:60-61: WARNING opportunity for
kobj_to_dev()

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>