regulator: qcom_spmi: Refactor get_mode/set_mode

spmi_regulator_common_get_mode and spmi_regulator_common_set_mode use
multi-level ifs which mirror a switch statement.  Refactor to use a switch
statement to make the code flow more clear.

Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>