UBI: fastmap break out of used PEB search

While searching for PEB matches for each volume in the used PEB list,
the search fails to stop when the PEB is found.  This patch adds
a break in the inner loop to stop the search when it is matched.

Signed-off-by: Brian Pomerantz <bapper@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>