staging: mfd: hi6421-spmi-pmic: deal with non-static functions

Several functions aren't used outside the mfd driver. So,
either remove or make them static.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d26323ab7be5b4efb73b1950b4daaa5959277449.1597647359.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>