mtd: rawnand: mtk: Fix wrongly assigned OOB buffer pointer issue

One main goal of the function mtk_nfc_update_ecc_stats is to check
whether sectors are all empty. If they are empty, set these sectors's
data buffer and OOB buffer as 0xff.

But now, the sector OOB buffer pointer is wrongly assigned. We always
do memset from sector 0.

To fix this issue, pass start sector number to make OOB buffer pointer
be properly assigned.

Fixes: 1d6b1e46 ("mtd: mediatek: driver for MTK Smart Device")
Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>