ASoC: sprd: use asoc_substream_to_rtd()

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

Signed-off-by: Kuninori Morimoto <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/sprd/sprd-pcm-dma.c

···
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct sprd_pcm_dma_private *dma_private = runtime->private_data;
-	struct snd_soc_pcm_runtime *rtd = substream->private_data;
+	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
 	struct sprd_pcm_dma_params *dma_params;
 	size_t totsize = params_buffer_bytes(params);
 	size_t period = params_period_bytes(params);