regulator: max77857: change some variables to static

max77857_regmap_config and max77857_driver are only used
in max77857-regulator.c now, change them to static.

Fixes: af71ccca ("regulator: max77857: Add ADI MAX77857/59/MAX77831 Regulator Support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230801130354.552243-1-yangyingliang@huawei.com


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