mtd: rawnand: sunxi: Precompute the ECC_CTL register value

The value computed by this function never changes for a given chip.
Compute the whole register value once up front, instead of every time
the ECC engine is enabled.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230204143520.9682-4-samuel@sholland.org