ASoC: xtensa: 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/xtensa/xtfpga-i2s.c

···
 			   struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
-	struct snd_soc_pcm_runtime *rtd = substream->private_data;
+	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
 	void *p;
 
 	snd_soc_set_runtime_hwparams(substream, &xtfpga_pcm_hardware);