iio: adc: stm32-adc: skip adc-diff-channels setup if none is present

If no adc differential channels are defined driver will fail with EINVAL:
stm32-adc: probe of 48003000.adc:adc@0 failed with error -22

Fix this by skipping the initialization if no channels are defined.

This applies only to the legacy way of initializing adc channels.

Fixes: d7705f35 ("iio: adc: stm32-adc: convert to device properties")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20230503162029.3654093-1-sean@geanix.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -2006,9 +2006,7 @@ static int stm32_adc_get_legacy_chan_count(struct iio_dev *indio_dev, struct stm
 	 * to get the *real* number of channels.
 	 */
 	ret = device_property_count_u32(dev, "st,adc-diff-channels");
-	if (ret < 0)
-		return ret;
-
+	if (ret > 0) {
 		ret /= (int)(sizeof(struct stm32_adc_diff_channel) / sizeof(u32));
 		if (ret > adc_info->max_channels) {
 			dev_err(&indio_dev->dev, "Bad st,adc-diff-channels?\n");
@@ -2017,6 +2015,7 @@ static int stm32_adc_get_legacy_chan_count(struct iio_dev *indio_dev, struct stm
 			adc->num_diff = ret;
 			num_channels += ret;
 		}
+	}
 
 	/* Optional sample time is provided either for each, or all channels */
 	adc->nsmps = device_property_count_u32(dev, "st,min-sample-time-nsecs");