powerpc/pseries/cmm: Report errors when registering notifiers fails

If we don't set the rc, we will return "0", making it look like we
succeeded.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-3-david@redhat.com