mtd: spinand: Implement mtd->_max_bad_blocks

We just have to use nanddev_mtd_max_bad_blocks().

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>