mtd: nand: fix chances to create incomplete ECC data when writing

When mtk_nfc_do_write_page() comparing the sector number,because the
sector number field is at the 12th-bit position of NFI_BYTELEN
register,the masked register should be shifted 12 bits before being
compared.The result of this bug may cause the second subpage has
incomplete ECC parity bytes.

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>