mtd: rawnand: fsl_elbc: Use ECC configuration from device tree

Initialize ECC configuration after nand_scan_ident() call and only in case
nand_scan_ident() have not done it. nand_scan_ident() fills ECC
configuration from device tree.

Fixes usage of NAND_ECC_SOFT_BCH when it is specified in device tree.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>