regulator: da9121: Mark some symbols with static keyword

Fix the following sparse warnings:

drivers/regulator/da9121-regulator.c:55:21: warning: symbol 'da9121_10A_2phase_current' was not declared. Should it be static?
drivers/regulator/da9121-regulator.c:63:21: warning: symbol 'da9121_6A_2phase_current' was not declared. Should it be static?
drivers/regulator/da9121-regulator.c:71:21: warning: symbol 'da9121_5A_1phase_current' was not declared. Should it be static?
drivers/regulator/da9121-regulator.c:79:21: warning: symbol 'da9121_3A_1phase_current' was not declared. Should it be static?
drivers/regulator/da9121-regulator.c:151:32: warning: symbol 'status_event_handling' was not declared. Should it be static?

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Acked-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/1606994795-36182-1-git-send-email-zou_wei@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>