mtd: nand: Fix memory allocation in nanddev_bbt_init()

Fix the size of the buffer allocated to store the in-memory BBT.
This bug was previously hidden by a different bug, that was fixed in
commit d098093b ("mtd: nand: Fix nanddev_neraseblocks()").

Fixes: 9c3736a3 ("mtd: nand: Add core infrastructure to deal with NAND devices")
Cc: <stable@vger.kernel.org>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>