power: supply: axp288_charger: Replace the extcon API

This patch uses the resource-managed extcon API for extcon_register_notifier()
and replaces the deprecated extcon API as following:
- extcon_get_cable_state_() -> extcon_get_state()

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>