ASoC: kirkwood: Use snd_soc_substream_to_rtd() for accessing private_data

Do not open-code snd_soc_substream_to_rtd().
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240430-asoc-snd-substream-clean-v1-8-6f8a8902b479@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/kirkwood/kirkwood-dma.c

@@ -19,7 +19,7 @@
 
 static struct kirkwood_dma_data *kirkwood_priv(struct snd_pcm_substream *subs)
 {
-	struct snd_soc_pcm_runtime *soc_runtime = subs->private_data;
+	struct snd_soc_pcm_runtime *soc_runtime = snd_soc_substream_to_rtd(subs);
 	return snd_soc_dai_get_drvdata(snd_soc_rtd_to_cpu(soc_runtime, 0));
 }