iio: adc: stm32-adc: add debug info if dt uses legacy channel config

Since nearly all stm32 dt's are using the legacy adc channel config,
we should warn users about using it.
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20230421083858.2613289-1-sean@geanix.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -1993,6 +1993,8 @@ static int stm32_adc_get_legacy_chan_count(struct iio_dev *indio_dev, struct stm
 	const struct stm32_adc_info *adc_info = adc->cfg->adc_info;
 	int num_channels = 0, ret;
 
+	dev_dbg(&indio_dev->dev, "using legacy channel config\n");
+
 	ret = device_property_count_u32(dev, "st,adc-channels");
 	if (ret > adc_info->max_channels) {
 		dev_err(&indio_dev->dev, "Bad st,adc-channels?\n");