ALSA: hda - Fix NULL dereference from CA0132 DSP loader
The CA0132 DSP loader leads to NULL deference since the recent
transition to HDA core code, as it unconditionally accesses
hdac_stream->substream->runtime. For DSP loading, the substream
shouldn't be assigned.
This patch addresses the NULL dereference above in addition to assure
the substream is cleared while DSP loading.