iio: adc: ad7768-1: use devm_regulator_get_enable_read_voltage()

iio: adc: ad7768-1: use devm_regulator_get_enable_read_voltage()

Use devm_regulator_get_enable_read_voltage() function as a standard and
concise way of reading the voltage from the regulator and keep the
regulator enabled. Replace the regulator descriptor with the direct
voltage value in the device struct.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/20250824041003.9727-1-Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>