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.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=98151


Signed-off-by: Takashi Iwai <tiwai@suse.de>