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>
diff --git a/sound/pci/hda/patch_sigmatel.c b/sound/pci/hda/patch_sigmatel.c
index d2ddb95..7f76a97 100644
--- a/sound/pci/hda/patch_sigmatel.c
+++ b/sound/pci/hda/patch_sigmatel.c
@@ -2689,7 +2689,7 @@
 		return NULL;
 	}
 	if (nid)
-		knew->subdevice = (1<<31)|nid;
+		knew->subdevice = HDA_SUBDEV_NID_FLAG | nid;
 	return knew;
 }