iio: dac: ad5592r: Get rid of OF specifics

Use dev_fwnode() instead of direct OF node dereference when checking
for regulator API error code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220531135320.63096-1-andriy.shevchenko@linux.intel.com


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