hda-audio: fix non-mixer codecs

They don't advertise mixer support, but still allow the guest change
mixer settings.  Add a check to avoid it.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/audio/hda-codec.c

@@ -261,6 +261,9 @@ static void hda_audio_set_amp(HDAAudioStream *st)
     left = left * 255 / QEMU_HDA_AMP_STEPS;
     right = right * 255 / QEMU_HDA_AMP_STEPS;
 
+    if (!st->state->mixer) {
+        return;
+    }
     if (st->output) {
         AUD_set_volume_out(st->voice.out, muted, left, right);
     } else {