iio: adc: mcp3564: use devm_regulator_get_enable_read_voltage()

iio: adc: mcp3564: use devm_regulator_get_enable_read_voltage()

This makes use of the new devm_regulator_get_enable_read_voltage()
helper function to reduce boilerplate code in the MCP3564 ADC driver.

The error message is slightly changed since there are fewer error
return paths.

Setting adc->vref_mv is consolidated into a single place to make the
logic easier to follow.

A use_internal_vref_attr local variable is added to make it more
obvious what the difference between the two iio info structures is.

The return value of the "Unknown Vref" dev_err_probe() is hard-coded to
-ENODEV instead of ret since it was getting a bit far from where ret
was set and logically that is the only value it could have.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marius Cristea <marius.cristea@microchip.com>
Link: https://patch.msgid.link/20240723-iio-regulator-refactor-round-3-v2-1-ae9291201785@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>