mtd: tests: ignore corrected bitflips in OOB on mtd_readtest

read_oob may now return ECC error codes. If the code is -EUCLEAN, then
we can safely ignore the error as a corrected bitflip.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>