mtd: nand: add mtd->writebufsize initialization

Initialize mtd->writebufsize to be equal to mtd->writesize.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>