mtd: fix the conversion from dev to mtd_info

The patch fixes a bug when converting dev to mtd_info by using the
drvdata of the dev, the previous code used
container_of(dev, struct mtd_info, dev), but won't work for the mtdXro
devices as they created without being contained inside mtd_info structure.

Signed-off-by: Saeed Bishara <saeed@marvell.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>