staging: spmi: hisi-spmi-controller: do some code cleanups

There are several minor things that can be cleanup in
order to make this driver more prepared for leaving staging.
Suggested-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1fa33567e21f95942d901a299d92e434d4a24b8f.1597647359.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>