mtd: use MTD_OPS_PLACE_OOB macro consistently

Use the MTD_OPS_PLACE_OOB to replace the hard code "0".
Make the code more readable.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>