power: supply: sc2731_charger: Add charger status detection

The USB charger status can be notified before the charger driver registers
the USB phy notifier, so we should check the charger status in probe() in
case we missed the USB charger notification.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>