iio: frequency: admv1013: Use devm_regulator_get_enable_read_voltage()

iio: frequency: admv1013: Use devm_regulator_get_enable_read_voltage()

We can reduce boilerplate code by using
devm_regulator_get_enable_read_voltage().

The common mode voltage is now passed as a parameter in the init
functions so we can avoid adding a state member that is only used
during init.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240429-regulator-get-enable-get-votlage-v2-5-b1f11ab766c1@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>