regulator: act8945a-regulator: make symbol act8945a_pm static

Fixes the following sparse warning:

drivers/regulator/act8945a-regulator.c:340:1: warning:
 symbol 'act8945a_pm' was not declared. Should it be static?

Fixes: 7482d6ec ("regulator: act8945a-regulator: Implement PM functionalities")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>