staging: regulator: hi6421v600-regulator: convert to use get/set voltage_sel

As the supported LDOs on this driver are all using a selector,
change the implementation to use get_voltage_sel and
set_voltage_sel ops.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1cf4f26540e65ffbb561c7d52e53f6be5bd63ac4.1597647359.git.mchehab+huawei@kernel.org


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