iio: adc: stm32: Fixing err code to not indicate success

This path would result in returning 0 / success on an error path.

Cc: Olivier Moysan <olivier.moysan@foss.st.com>
Fixes: 95bc8184 ("iio: adc: stm32-adc: add support of generic channels binding")
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240330185305.1319844-4-jic23@kernel.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -2234,6 +2234,7 @@ static int stm32_adc_generic_chan_init(struct iio_dev *indio_dev,
 			if (vin[0] != val || vin[1] >= adc_info->max_channels) {
 				dev_err(&indio_dev->dev, "Invalid channel in%d-in%d\n",
 					vin[0], vin[1]);
+				ret = -EINVAL;
 				goto err;
 			}
 		} else if (ret != -EINVAL) {