iio: dac: ad5446: Don't ignore missing regulator

iio: dac: ad5446: Don't ignore missing regulator

If the chip does not have an internal reference, do not ignore a missing
regulator as we won't be able to actually provide a proper scale for the
DAC. Since it's now seen as an error, flip the if() logic so errors are
treated first (which is the typical pattern).

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>