mtd: spinand: Fix OOB read

So far OOB have never been used in SPI-NAND, add the missing memcpy to
make it work properly.

Fixes: 7529df46 ("mtd: nand: Add core infrastructure to support SPI NANDs")
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201001102014.20100-6-miquel.raynal@bootlin.com