mtd: rawnand: Use the ECC framework nand_ecc_is_strong_enough() helper

Plus, the new helper has a more "english" name.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-19-miquel.raynal@bootlin.com