mtd/fsl_elbc: take NAND_ECC_SOFT_BCH config option into account

NAND_ECC_SOFT was the only option available while the SOFT_BCH option
may also be used.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Acked-by: Scott Wood <scottwood@freescale.com>