power: supply: sc2731_charger: Add one work to charge/discharge

Since the USB notifier context is atomic, we can not start or stop charging
in atomic context. Thus this patch adds one work to help to charge or
discharge.

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