hwmon: (adc128d818) Use devm_regulator_get_enable_read_voltage()

hwmon: (adc128d818) Use devm_regulator_get_enable_read_voltage()

We can reduce boilerplate code and eliminate the driver remove()
function by using devm_regulator_get_enable_read_voltage().

A new external_vref flag is added since we no longer have the handle
to the regulator to check if it is present.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240429-regulator-get-enable-get-votlage-v2-2-b1f11ab766c1@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>