In 'nor_erase_prepare()' we want to make sure the UBI headers are
corrupted. But it is possible that one of the headers just contains
all 0xFFs, which is also OK, because UBI will erase it in case of
a power cut.
Signed-off-by: NHolger Brunck <holger.brunck@keymile.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>