staging: hikey9xx: hi6421v600-regulator: use some regmap helpers

Now that the driver was ported to use regmap, let's use
some help functions in order to simplify the code a little
bit.

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/5bdd7988fb9bf5d3ebee03724cf73cc04d9bc47b.1611949675.git.mchehab+huawei@kernel.org


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