ASoC: soc-xxx: add asoc_substream_to_rtd()

Current soc-xxx are getting rtd from substream by

	rtd = substream->private_data;

But, getting data from "private_data" is very unclear.
This patch adds asoc_substream_to_rtd() macro which is
easy to understand that rtd from substream.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87wo2z0yve.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/soc-utils.c b/sound/soc/soc-utils.c
index 364b248..f27f94c 100644
--- a/sound/soc/soc-utils.c
+++ b/sound/soc/soc-utils.c
@@ -66,7 +66,7 @@ static const struct snd_pcm_hardware dummy_dma_hardware = {
 static int dummy_dma_open(struct snd_soc_component *component,
 			  struct snd_pcm_substream *substream)
 {
-	struct snd_soc_pcm_runtime *rtd = substream->private_data;
+	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
 
 	/* BE's dont need dummy params */
 	if (!rtd->dai_link->no_pcm)