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>

sound/pci/hda/patch_analog.c

@@ -1092,6 +1092,7 @@ static int patch_ad1884(struct hda_codec *codec)
 	spec = codec->spec;
 
 	spec->gen.mixer_nid = 0x20;
+	spec->gen.mixer_merge_nid = 0x21;
 	spec->gen.beep_nid = 0x10;
 	set_beep_amp(spec, 0x10, 0, HDA_OUTPUT);