mtd: nand: fix generating over-boundary ECC data when writing

When mtk_ecc_encode() is writing the ECC parity data to the OOB
region,because each register is 4 bytes in length,but the len's unit is
in bytes,the operation in the for loop will cross the ECC's boundary.

Signed-off-by: RogerCC Lin <rogercc.lin@mediatek.com>
Fixes: 1d6b1e46 ("mtd: mediatek: driver for MTK Smart Device")
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>