mtd_blkdevs: simplify blktrans_getgeo

No need to grab a mtd_blktrans_dev given that ->open already holds one
and ->getgeo can only be called on an open disk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210823073359.705281-8-hch@lst.de