staging: mfd: hi6421-spmi-pmic: cleanup the code

There are several small cleanups that can be done in order to
make the code more prepared to be upstreamed.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/823792ba2f69e613629ab52a33e5728d54e2288b.1597647359.git.mchehab+huawei@kernel.org


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