mtd: eLBC NAND: fix subpage write support

As subpage write is enabled by default for all drivers, nand_write_subpage_hwecc
causes a crash if the driver did not register ecc->hwctl or ecc->calculate.
This behavior was introduced in
   commit 837a6ba4
   "mtd: nand: subpage write support for hardware based ECC schemes".

This fixes a crash by emulating subpage write support by padding sub-page data
with 0xff on either sides to make it full page compatible.

Reported-by: Helmut Schaa <helmut.schaa@googlemail.com>
Tested-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Scott Wood <scottwood@freescale.com>
Cc: <stable@vger.kernel.org> # 3.10.x+
Signed-off-by: Brian Norris <computersforpeace@gmail.com>