UBI: fix the volumes tree sorting criteria

Commig "604b592e UBI: fix rb_tree node comparison in add_map"
broke fastmap backward compatibility and older fastmap images
cannot be mounted anymore. The reason is that it changes the
volumes RB-tree sorting criteria. This patch fixes the problem.

Artem: re-write the commit message
Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/fastmap.c

@@ -125,7 +125,7 @@ static struct ubi_ainf_volume *add_vol(struct ubi_attach_info *ai, int vol_id,
 		parent = *p;
 		av = rb_entry(parent, struct ubi_ainf_volume, rb);
 
-		if (vol_id < av->vol_id)
+		if (vol_id > av->vol_id)
 			p = &(*p)->rb_left;
 		else
 			p = &(*p)->rb_right;