mtd: nand: omap: fix ecclayout->oobfree->length

This patch excludes reserved-marker byte-position from oobfree->length
calculation. Thus all bytes from oobfree->offset till end of OOB are free.

CC: <stable@vger.kernel.org> # 3.13.x+
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>