mtd: rawnand: bbt: Skip bad blocks when searching for the BBT in NAND

The blocks containing the bad block table can become bad as well. So
make sure to skip any blocks that are marked bad when searching for the
bad block table.

Otherwise in very rare cases where two BBT blocks wear out it might
happen that an obsolete BBT is used instead of a newer available
version.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210325102337.481172-1-s.riedmueller@phytec.de