ASoC: mediatek: mt8192: use asoc_substream_to_rtd()

Uses asoc_substream_to_rtd() helper.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20210203032201.2882158-3-tzungbi@google.com


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