mtd: sh_flctl: Only copy OOB data if it is required

Check the new oob_required flag and only copy the OOB data to the internal
buffer if needed.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>