mtd: grab a reference to the MTD of_node before registering it

We now stick the device node representing the current MTD (if any) into
sysfs, so let's make sure we have a reference to it before doing that.

Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>