mtd: Better comment NAND_BBT_NO_OOB

Amend the comment to reflect the fact NAND_BBT_NO_OOB refers to the
location of the bad block table marker.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>