soundwire: Use snd_soc_substream_to_rtd() to obtain rtd

Utilize the helper function instead of casting from ->private_data
directly.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Link: https://lore.kernel.org/r/20240219105206.335738-1-cezary.rojewski@intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>