mtd: mtd_oobtest: Handle bitflips during reads

[ Upstream commit 12663b442e5ac5aa3d6097cd3f287c71ba46d26e ]

Reads from NAND devices usually trigger bitflips, this is an expected
behavior. While bitflips are under a given threshold, the MTD core
returns 0. However, when the number of corrected bitflips is above this
same threshold, -EUCLEAN is returned to inform the upper layer that this
block is slightly dying and soon the ECC engine will be overtaken so
actions should be taken to move the data out of it.

This particular condition should not be treated like an error and the
test should continue.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>