powerpc/pseries/cmm: Implement release() function for sysfs device

When unloading the module, one gets
  ------------[ cut here ]------------
  Device 'cmm0' does not have a release() function, it is broken and must be fixed. See Documentation/kobject.txt.
  WARNING: CPU: 0 PID: 19308 at drivers/base/core.c:1244 .device_release+0xcc/0xf0
  ...

We only have one static fake device. There is nothing to do when
releasing the device (via cmm_exit()).

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-2-david@redhat.com