ASoC: sunxi: 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>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20240430-asoc-snd-substream-clean-v1-13-6f8a8902b479@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sunxi/sun50i-dmic.c

@@ -86,7 +86,7 @@ static const struct dmic_rate dmic_rate_s[] = {
 static int sun50i_dmic_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 = snd_soc_substream_to_rtd(substream);
 	struct sun50i_dmic_dev *host = snd_soc_dai_get_drvdata(snd_soc_rtd_to_cpu(rtd, 0));
 
 	/* only support capture */