ALSA: hda - Get rid of magic digits for subdev hack

Define a proper const for a magic 31bit flag for subdev / NID setup
with a brief comment.

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

sound/pci/hda/patch_realtek.c

@@ -4324,7 +4324,7 @@ static int add_control(struct alc_spec *spec, int type, const char *name,
 	if (!knew->name)
 		return -ENOMEM;
 	if (get_amp_nid_(val))
-		knew->subdevice = (1<<31)|get_amp_nid_(val);
+		knew->subdevice = HDA_SUBDEV_NID_FLAG | get_amp_nid_(val);
 	knew->private_value = val;
 	return 0;
 }