ALSA: hdac: Add macro for hda ext devices entry

With the new modalias infrastructure support added for hda, create a macro
for ext devices similar to legacy to add the device entry.

Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>