ASoC: SOF: Correct sps->stream and cstream nullity

Merge series from Peter Ujfalusi <peter.ujfalusi@linux.intel.com>:

The Nullity of sps->cstream needs to be checked in sof_ipc_msg_data() and not
assume that it is not NULL.
The sps->stream must be cleared to NULL on close since this is used as a check
to see if we have active PCM stream.