mtd: rawnand: txx9ndfmc: clarify ECC parameters assignation

A comment in the probe declares that values are assigned to ecc.size
and ecc.bytes, but these values will be overwritten. This is not
entirely right as they are overwritten only if
mtd->writesize >= 512. Let's clarify this by moving these assignations
to txx9ndfmc_nand_scan().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>