mtd: nand: ignore ECC errors for simple BBM scans

Now that nand_do_readoob() may return -EUCLEAN or -EBADMSG on ECC errors,
we need to handle the return value specially in some cases.

When scanning for simple bad block markers, reacting to an ECC error is
not very useful, as we assume that the relevant markers are still
non-0xFF for true bad blocks.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>