ASoC: Intel: avs: Use asoc_substream_to_rtd() to obtain rtd

Utilize the helper function instead of casting from ->private_data
or snd_pcm_substream_chip() directly.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Link: https://lore.kernel.org/r/20230113191410.1454566-1-amadeuszx.slawinski@linux.intel.com


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