mtd: nand: read ECC algorithm from the new field

Now we have all drivers properly setting this new field we can start
using it. For a very short period of time we should support both values:
NAND_ECC_SOFT and NAND_ECC_SOFT_BCH treating them the same. It's because
of_get_nand_ecc_mode may still be setting NAND_ECC_SOFT_BCH.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>