UBI: fix the volumes tree sorting criteria

Commig "604b592 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>