power: supply: sc27xx: Make sure the alarm capacity is larger than 0

We must make sure the alarm capacity is larger than 0, to help to
calibrate the low battery capacity.

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