staging: hikey9xx: hi6421v600-regulator: do some cleanups

staging: hikey9xx: hi6421v600-regulator: do some cleanups

In preparation for de-staging, do some cleanups:

- Return error codes from hi6421_spmi_pmic_rmw();
- Remove a debug message;
- Change the module description;
- a few minor coding style adjustments.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0bae0c05d997e4a5a0b3b86a65f3370dafb14596.1611212783.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>