snd_sof_pcm_stream keeps information about both PCM (snd_pcm_substream)
and Compress (snd_compr_stream) streams.

When PCM substream pointer is NULL this means we are dealing with a
compress stream.
Reviewed-by: Paul Olaru <paul.olaru@nxp.com>
Reviewed-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20230117122533.201708-4-daniel.baluta@oss.nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>