regulator: hi6421: mark hi6421_regulator_ldo_get_optimum_mode() static

We get 1 warning when building kernel with W=1:
drivers/regulator/hi6421-regulator.c:480:14: warning: no previous prototype for
'hi6421_regulator_ldo_get_optimum_mode' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>