mtd: nand: fsmc_nand: handle on-die ECC case

This commit adjusts the fsmc_nand driver so that it accepts the
NAND_ECC_ON_DIE case. It simply does nothing in this case, since both
the ECC operations and OOB layout will be defined by the NAND chip code
rather than by the NAND controller code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>