mtd: call the remove notifiers before assuming it is in use

Now that mtd block common layer is prepared for proper hotplug support,
enable it here

Now all users of the mtd device have a chance to put the mtd device
when they are notified to do so, and they have to do so to make hotplug work.

[dwmw2: There's more work to be done to fix hotplug in the general case, but
        this is a reasonable start]

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>