mtd: rawnand: Get rid of chip->ecc_{strength,step}_ds

nand_device embeds a nand_ecc_req object which contains the minimum
strength and step-size required by the NAND device.

Drop the chip->ecc_{strength,step}_ds fields and use
chip->base.eccreq.{strength,step_size} instead.

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>