staging: unisys: Fix clean up path

When unloading a module, we need to cleanup the platform registration.
However, unregistering the platform uncovered a couple of quirks, namely
a missing device_release function.  Fix things up so module unload works
and allows us to reload the module.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>