iio: adc: ad7192: use devm_regulator_get_enable_read_voltage

iio: adc: ad7192: use devm_regulator_get_enable_read_voltage

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

Error messages have changed slightly since there are now fewer places
where we print an error. The rest of the logic of selecting which
supply to use as the reference voltage remains the same.

Also 1000 is replaced by MILLI in a few places for consistency.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240624124941.113010-2-alisa.roman@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>