mtd: rawnand: sunxi: Fix the size of the last OOB region

The previous code assigned to the wrong structure member.

Fixes: c66811e6

 ("mtd: nand: sunxi: switch to mtd_ooblayout_ops")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-By: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221229181526.53766-6-samuel@sholland.org