mtd: rfd_ftl: use container_of() rather than cast

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

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210807214538.14484-6-sean@mess.org