regulator: max8998: Staticify internal function max8998_get_current_limit()

max8998_get_current_limit() is only used via the .get_current_limit,
so it doesn't need to be publicly supported, or to have its own
external prototype.  Instead, we'll make it static.

Fixes the following W=1 warning:

 drivers/regulator/max8998.c:418:5: warning: no previous prototype for ‘max8998_get_current_limit’ [-Wmissing-prototypes]
 418 | int max8998_get_current_limit(struct regulator_dev *rdev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200626065738.93412-2-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>