mtd: mtd: drop NAND_ECC_SOFT_BCH enum value

This value should not be part of nand_ecc_modes_t as it specifies
algorithm not a mode. We successfully managed to introduce new "algo"
field which is respected now.

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