ASoC: stm32: dfsdm: add stm32_adfsdm_dummy_cb() callback

Adapt STM32 DFSDM driver to a change in iio_channel_get_all_cb() API.
The callback pointer becomes a requested parameter of this API,
so add a dummy callback to be given as parameter of this function.
However, the stm32_dfsdm_get_buff_cb() API is still used instead,
to optimize DMA transfers.
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20201121161457.957-2-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

sound/soc/stm/stm32_adfsdm.c

@@ -293,6 +293,16 @@ static int stm32_adfsdm_pcm_new(struct snd_soc_component *component,
 	return 0;
 }
 
+static int stm32_adfsdm_dummy_cb(const void *data, void *private)
+{
+	/*
+	 * This dummmy callback is requested by iio_channel_get_all_cb() API,
+	 * but the stm32_dfsdm_get_buff_cb() API is used instead, to optimize
+	 * DMA transfers.
+	 */
+	return 0;
+}
+
 static struct snd_soc_component_driver stm32_adfsdm_soc_platform = {
 	.open		= stm32_adfsdm_pcm_open,
 	.close		= stm32_adfsdm_pcm_close,
@@ -335,7 +345,7 @@ static int stm32_adfsdm_probe(struct platform_device *pdev)
 	if (IS_ERR(priv->iio_ch))
 		return PTR_ERR(priv->iio_ch);
 
-	priv->iio_cb = iio_channel_get_all_cb(&pdev->dev, NULL, NULL);
+	priv->iio_cb = iio_channel_get_all_cb(&pdev->dev, &stm32_adfsdm_dummy_cb, NULL);
 	if (IS_ERR(priv->iio_cb))
 		return PTR_ERR(priv->iio_cb);