ASoC: amd: use asoc_substream_to_rtd()

Uses asoc_substream_to_rtd() helper.

Signed-off-by: Zhen Ni <nizhen@uniontech.com>
Link: https://lore.kernel.org/r/20220302081502.25367-1-nizhen@uniontech.com


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