Fix the following sparse warning:

drivers/power/supply/ucs1002_power.c:492:5: warning: symbol 'ucs1002_regulator_enable' was not declared. Should it be static?

Fixes: a3d70dac ("power: suppy: ucs1002: disable power when max current is 0")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>