mtd: nand: mtk: fix incorrect register setting order about ecc irq

Currently, we trigger ECC HW before setting ecc irq. It is incorrect.
Because ECC starts working once the register ECC_CTL_REG is set as
ECC_OP_ENABLE. And this may lead an abnormal behavior of ecc irq.
So, should enable ecc irq at first, then trigger ECC.

Fixes: 1d6b1e46 ("mtd: mediatek: driver for MTK Smart Device")
Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>