staging:iio:dac:ad5504 dereferencing just freed st in error path.

Introduced when I did the iio_chan_spec conversion. Detected by smatch.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/dac/ad5504.c

@@ -338,7 +338,7 @@ static int __devinit ad5504_probe(struct spi_device *spi)
 	iio_free_device(indio_dev);
 error_disable_reg:
 	if (!IS_ERR(reg))
-		regulator_disable(st->reg);
+		regulator_disable(reg);
 error_put_reg:
 	if (!IS_ERR(reg))
 		regulator_put(reg);