mtd: nand: tests: fix regression introduced in mtd_nandectest

Offending Commit: 6e941192 "mtd: nand: return consistent error codes in
ecc.correct() implementations"

The new error code was not being handled properly in double bit error
detection.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>