iio: addac: ad74413r: use devm_regulator_get_enable_read_voltage()

iio: addac: ad74413r: use devm_regulator_get_enable_read_voltage()

It's highly unlikely for the converter ref voltage to change at runtime.
Hence, let's read the voltage and save it (instead of the regulator
struct). While at it, simplify the code by using
devm_regulator_get_enable_read_voltage().

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20241016-dev-ad74413r-minor-improv-v1-2-13c9c769237d@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>