mtd: rawnand: marvell: rework BCH engine failure path

We are about to support a new layout that triggers a faulty mechanism in
BCH engine that creates bitflips in erased pages.

Before adding the quirk that will workaround this issue, this patch just
reworks a bit the section that handles ECC failures in BCH read path.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>