iio: dac: ad5686: use devm_regulator_get_enable_read_voltage()

iio: dac: ad5686: use devm_regulator_get_enable_read_voltage()

Simplify the code by using devm_regulator_get_enable_read_voltage().

There is a small change in behavior. Before, all errors from
devm_regulator_get_optional() were ignored and assumed to mean that
the external reference supply was absent. Now, only -ENODEV is checked
and other errors will cause a failure to probe. So now, this will
catch errors, like using the wrong data type for the devicetree
property.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241120-iio-regulator-cleanup-round-6-v1-3-d5a5360f7ec3@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>