mtd: ftl: use container_of() rather than cast

The container_of() is much more readable and also safer.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220816135910.268016-1-cuigaosheng1@huawei.com