iio: adc: ltc2309: use devm_regulator_get_enable_read_voltage()

iio: adc: ltc2309: use devm_regulator_get_enable_read_voltage()

Use devm_regulator_get_enable_read_voltage() to simplify the code.
Error message is changed since there is only one error return now.
LTC2309_INTERNAL_REF_MV macro is added to make the internal reference
voltage value self-documenting.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240621-iio-regulator-refactor-round-2-v1-5-49e50cd0b99a@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>