iio: adc: imx93: fix a signedness bug in imx93_adc_read_raw()

The problem is these lines:

	ret = vref_uv = regulator_get_voltage(adc->vref);
	if (ret < 0)

The "ret" variable is type long and "vref_uv" is u32 so that means
the condition can never be true on a 64bit system.  A negative error
code from regulator_get_voltage() would be cast to a high positive
u32 value and then remain a high positive value when cast to a long.

The "ret" variable only ever stores ints so it should be declared as
an int.  We can delete the "vref_uv" variable and use "ret" directly.

Fixes: 7d02296a ("iio: adc: add imx93 adc support")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://lore.kernel.org/r/Y+utEvjfjQRQo2QB@kiliSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/imx93_adc.c

@@ -236,8 +236,7 @@ static int imx93_adc_read_raw(struct iio_dev *indio_dev,
 {
 	struct imx93_adc *adc = iio_priv(indio_dev);
 	struct device *dev = adc->dev;
-	long ret;
-	u32 vref_uv;
+	int ret;
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
@@ -253,10 +252,10 @@ static int imx93_adc_read_raw(struct iio_dev *indio_dev,
 		return IIO_VAL_INT;
 
 	case IIO_CHAN_INFO_SCALE:
-		ret = vref_uv = regulator_get_voltage(adc->vref);
+		ret = regulator_get_voltage(adc->vref);
 		if (ret < 0)
 			return ret;
-		*val = vref_uv / 1000;
+		*val = ret / 1000;
 		*val2 = 12;
 		return IIO_VAL_FRACTIONAL_LOG2;