mtd: move mtd_read_oob() definition out of mtd.h

mtd_read_oob() will be expanded a little, so don't leave it in the header
as a static inline function.

Signed-off-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>