regulator: anatop: Remove checking control_reg in [set|get]_voltage_sel

Remove checking control_reg in [set|get]_voltage_sel and then convert to use
regulator_[set|get]_voltage_sel_regmap for [set|get]_voltage_sel callbacks.

The anatop-reg-offset property is a required property rather than optional
property. So the question is what is the meaning of setting anatop-reg-offset
to 0? If 0 is a valid setting for anatop-reg-offset and it has special meaning,
we had better document it in the binding document. Otherwise, remove the testing
for control_reg in the driver.

No anatop voltage regulator node in the dts files set anatop-reg-offset to 0.
So I think it's safe to remove testing if control_reg is 0.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>