mfd: ene-kb3930: Make local symbol 'kb3930_power_off' static

The sparse tool complains as follows:

drivers/mfd/ene-kb3930.c:36:15: warning: symbol 'kb3930_power_off' was not declared. Should it be static?

The symbol is not used outside of this file.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>