iio: dac: ad5504: Don't set error code to voltage_uv

regulator_get_voltage() may return negative error code.
Add error checking to avoid setting error code to voltage_uv.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/dac/ad5504.c

@@ -296,7 +296,11 @@ static int __devinit ad5504_probe(struct spi_device *spi)
 		if (ret)
 			goto error_put_reg;
 
-		voltage_uv = regulator_get_voltage(reg);
+		ret = regulator_get_voltage(reg);
+		if (ret < 0)
+			goto error_disable_reg;
+
+		voltage_uv = ret;
 	}
 
 	spi_set_drvdata(spi, indio_dev);