mtd: spinand: Allow the case where there is no ECC engine

Even if this is not supposed to happen, there is no reason to fail the
probe if it was explicitly requested to use no ECC engine at all (for
instance, during development). This condition is met by just
commenting out the error on the OOB free bytes count after the
assignation of an ECC engine if none was provided (any other situation
would error out much earlier anyway).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201001102014.20100-5-miquel.raynal@bootlin.com