ALSA: hda - Add missing loopback merge path for AD1884/1984 codecs

The mixer widget (NID 0x20) of AD1884 and AD1984 codecs isn't
connected directly to the actual I/O paths but only via another mixer
widget (NID 0x21).  We need a similar fix as we did for AD1882.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>