iio: adc: ad7944: use devm_regulator_get_enable_read_voltage

This makes use of the new devm_regulator_get_enable_read_voltage()
function to reduce boilerplate code.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240612-iio-adc-ref-supply-refactor-v2-5-fa622e7354e9@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>