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 <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87v9ijyoek.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/xtensa/xtfpga-i2s.c

@@ -369,7 +369,7 @@ static int xtfpga_pcm_open(struct snd_soc_component *component,
 			   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);