mtd: drop support for NAND_ECC_SOFT_BCH as "soft_bch" mapping

There isn't any difference between handling NAND_ECC_SOFT and
NAND_ECC_SOFT_BCH now. What matters is the new field called "algo".
Please note we're keeping backward DT compatibility. We are still
treating "soft_bch" value as the one setting Hamming algorithm, it's
just handled in of_get_nand_ecc_algo now.

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