power: supply: axp288-charger: Simplify axp288_get_charger_health()

Now that axp288_charger_usb_update_property() reads and caches all
relevant registers, axp288_get_charger_health() can be simplified
by directly returning the health.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>