ASoC: fsl: 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-v1-6-6f8a8902b479@linaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>