staging: mfd: hi6421-spmi-pmic: get rid of the static vars

There are several static vars inside this driver.

Get rid of them.

While here, add a SPDX header file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/19c497fc2bb1d3a95863d92cac89869d5abe3f2e.1597647359.git.mchehab+huawei@kernel.org


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