mtd: nand: mtk: release lock on error path

We only want to hold the lock on the success path, not this error path.

Fixes: 7ec4a37c ("mtd: nand: mediatek: add support for different MTK NAND FLASH Controller IP")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>