mtd: rawnand: micron: Avoid enabling/disabling ECC when it can't be disabled

Some chips have their on-die ECC forcibly enabled, there's no point in
trying to enable/disable the ECC engine in that case.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>