mtd: nand: ecc-hamming: Let the software Hamming ECC engine be unselected

There is no reason to always embed the software Hamming ECC engine
implementation. By default it is (with raw NAND), but we can let the
user decide.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-19-miquel.raynal@bootlin.com