ALSA: hda: Fix possible null-ptr-deref when assigning a stream

While AudioDSP drivers assign streams exclusively of HOST or LINK type,
nothing blocks a user to attempt to assign a COUPLED stream. As
supplied substream instance may be a stub, what is the case when
code-loading, such scenario ends with null-ptr-deref.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Link: https://lore.kernel.org/r/20231006102857.749143-2-cezary.rojewski@intel.com


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