mtd: nand: ecc-hamming: Rename the exported functions

Prefix by ecc_sw_hamming_ the functions which should be internal only
but are exported for "raw" operations.

Prefix by nand_ecc_sw_hamming_ the other functions which will be used
in the context of the declaration of an Hamming proper ECC engine
object.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-16-miquel.raynal@bootlin.com