UBI: tighten the corrupted PEB criteria

If we get a bit-flip of ECC error while reading the data area, do not add it to
corrupted list, because it is possible that this is just unstable PEB with
corruptions caused by unclean reboots.

This patch also improves commentaries.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>