ASoC: mxs: 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/87tuy3yoef.wl-kuninori.morimoto.gx@renesas.com


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