iio: adc: mcp3911: 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 MCP3911 ADC driver.

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

An extra parameter is added to the config callback to avoid adding
state that is not used outside of the probe() function.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://patch.msgid.link/20240712-iio-regulator-refactor-round-3-v1-6-835017bae43d@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>