mtd: move mtd_oob_mode_t to shared kernel/user space

We will want to use the MTD_OOB_{PLACE,AUTO,RAW} modes in user-space
applications through the introduction of new ioctls, so we should make
this enum a shared type.

This enum is now anonymous.

Artem: tweaked the patch.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>