mtd: rawnand: Return an enum from of_get_nand_ecc_algo()

There is an enumeration to list ECC algorithm, let's use it instead of
returning an int.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200526195633.11543-6-miquel.raynal@bootlin.com