mtd_blkdevs: simplify blktrans_dev_get

->private_data is set before the disk is added and never cleared, so don't
bother trying to handle a NULL pointer there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210823073359.705281-6-hch@lst.de