mtd: OneNAND: add missing __devexit_p

Add missing __devexit_p wrapper and no more mark shutdown with __devexit.
Fixes build in configurations where devexit functions get discarded.

Signed-off-by: Mika Korhonen <mika.j.korhonen@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>