mtd: rawnand: Use nanddev_get/set_ecc_requirements() when relevant

Instead of accessing ->strength/step_size directly.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-15-miquel.raynal@bootlin.com