mtd: nand: mediatek: refine register NFI_PAGEFMT setting

The register NFI_PAGEFMT is always 32 bits length, so it is better to
do register program using writel() compare with writew().

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>