mtd: kill the ecclayout->oobavail field

ecclayout->oobavail is just redundant with the mtd->oobavail field.
Moreover, it prevents static const definition of ecc layouts since the
NAND framework is calculating this value based on the ecclayout->oobfree
field.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>