power: supply: axp288_charger: Use the defined variable to clean code

Use the defined variable "dev" to make the code cleaner.

Co-developed-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>