ASoC: sunxi: use asoc_substream_to_rtd()

Now we can use asoc_substream_to_rtd() macro,
let's use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87r1t70yuw.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/sunxi/sun4i-spdif.c b/sound/soc/sunxi/sun4i-spdif.c
index 326dd45..228485f 100644
--- a/sound/soc/sunxi/sun4i-spdif.c
+++ b/sound/soc/sunxi/sun4i-spdif.c
@@ -243,7 +243,7 @@ static void sun4i_snd_txctrl_off(struct snd_pcm_substream *substream,
 static int sun4i_spdif_startup(struct snd_pcm_substream *substream,
 			       struct snd_soc_dai *cpu_dai)
 {
-	struct snd_soc_pcm_runtime *rtd = substream->private_data;
+	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
 	struct sun4i_spdif_dev *host = snd_soc_dai_get_drvdata(asoc_rtd_to_cpu(rtd, 0));
 
 	if (substream->stream != SNDRV_PCM_STREAM_PLAYBACK)