mtd: fix callback return value check

Drivers may (and do) return negative errno values other than -1 from the
ecc.correct callback.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>