mtd: rawnand: Add an invalid ECC mode to discriminate with valid ones

NAND ECC modes (or providers) have their own enumeration but, unlike
their algorithms counterpart, there is no invalid or uninitialized
value to discriminate between an error and having chosen a no-ECC
situation. Add an "invalid" entry for this purpose.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200526195633.11543-7-miquel.raynal@bootlin.com