power: supply: surface-charger: Make symbol 'surface_ac_pm_ops' static

The sparse tool complains as follows:

drivers/power/supply/surface_charger.c:229:1: warning:
 symbol 'surface_ac_pm_ops' was not declared. Should it be static?

This symbol is not used outside of surface_charger.c, so this
commit marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Acked-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>