mtd: nand: gpmi: need to use {read,write}_oob_raw

This patch is simply an added warning in the comments. Ideally, this patch
need not be merged, but rather, a developer will write a proper solution
that can use the ecc.read_oob_raw and ecc.write_oob_raw interfaces.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>