iio: dac: mcp4922: 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 MCP4922 DAC driver.

The error message is slightly different because there is only one error
return path now.

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


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