Assume that:
          tmp = ((extid >> 2) & 0x04) | (extid & 0x03));

From the K9LCG08U0B's datasheet, we know that:
  the oob size is 640 when tmp is 6;
  the oob size is 1024 when tmp is 7;
Signed-off-by: Huang Shijie <b32955@freescale.com>
[Brian: fixed compile issue]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>