mtd: Fix typo in mtd_ooblayout_set_databytes() description

Fix a probable copy/paste error: the function works like
mtd_ooblayout_set_bytes(), not *_get_bytes().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200526195633.11543-2-miquel.raynal@bootlin.com