mtd: nand: gpmi: remove deprecated comment

Now that we have raw functions properly implemented we can remove this
FIXME.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>