mtd: nand: gpmi-nand: use more sensible error codes at various places

Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>