mtd/ftl: don't cast away the type when calling add_mtd_blktrans_dev

Pass the actual mtd_blktrans_dev instead of casting the containing
structure to void *.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210823073359.705281-4-hch@lst.de