iio: adc: stm32-dfsdm: add check on max filter id

reg property should be checked against number of available filters.
BTW, dfsdm->num_fls wasn't used. But it can be used for this purpose.
This prevents using data out of allocated dfsdm->fl_list array.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-dfsdm-adc.c

@@ -1108,8 +1108,8 @@ static int stm32_dfsdm_adc_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, adc);
 
 	ret = of_property_read_u32(dev->of_node, "reg", &adc->fl_id);
-	if (ret != 0) {
-		dev_err(dev, "Missing reg property\n");
+	if (ret != 0 || adc->fl_id >= adc->dfsdm->num_fls) {
+		dev_err(dev, "Missing or bad reg property\n");
 		return -EINVAL;
 	}