power: supply: sc27xx: prevent adc * 1000 from overflow

The input parameter is int type, cause adc * 1000 could overflow.
Change to use s64 to avoid this issue.

Signed-off-by: Chen Yongzhi <yongzhi.chen@unisoc.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>