iio: adc: aspeed_adc: use devm_regulator_get_enable_read_voltage()

iio: adc: aspeed_adc: use devm_regulator_get_enable_read_voltage()

This makes use of the devm_regulator_get_enable_read_voltage() helper
function to simplify the code.

The error return is moved closer to the function call to make it easier
to follow the logic. And a few blank lines are added for readability.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Link: https://patch.msgid.link/20240621-iio-regulator-refactor-round-2-v1-1-49e50cd0b99a@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>