iio: adc: imx8qxp-adc: propagate regulator_get_voltage error

If the ADC vref regulator returns an error, for example, if CONFIG_REGULATOR
is not set, the error will be used as a reference voltage.

Introduce a guard for negative return values instead of unconditionally
casting it to u32.
Acked-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Martin Larsson <martin.larsson@actia.se>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20220720153136.3502440-1-martin.larsson@actia.seSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/imx8qxp-adc.c

@@ -202,7 +202,7 @@ static int imx8qxp_adc_read_raw(struct iio_dev *indio_dev,
 	struct imx8qxp_adc *adc = iio_priv(indio_dev);
 	struct device *dev = adc->dev;
 
-	u32 ctrl, vref_uv;
+	u32 ctrl;
 	long ret;
 
 	switch (mask) {
@@ -245,8 +245,10 @@ static int imx8qxp_adc_read_raw(struct iio_dev *indio_dev,
 		return IIO_VAL_INT;
 
 	case IIO_CHAN_INFO_SCALE:
-		vref_uv = regulator_get_voltage(adc->vref);
-		*val = vref_uv / 1000;
+		ret = regulator_get_voltage(adc->vref);
+		if (ret < 0)
+			return ret;
+		*val = ret / 1000;
 		*val2 = 12;
 		return IIO_VAL_FRACTIONAL_LOG2;