ubi: Reject MLC NAND

While UBI and UBIFS seem to work at first sight with MLC NAND, you will
most likely lose all your data upon a power-cut or due to read/write
disturb.
In order to protect users from bad surprises, refuse to attach to MLC
NAND.

Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Artem Bityutskiy <dedekind1@gmail.com>