ASoC: Use snd_soc_substream_to_rtd() for accessing

Merge series from Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>:

Do not open-code snd_soc_substream_to_rtd() when accessing
snd_pcm_substream->private_data.  This makes code more consistent with
rest of ASoC and allows in the future to move the field to any other
place or add additional checks in snd_soc_substream_to_rtd().