powerpc/pseries/cmm: Cleanup rc handling in cmm_init()

No need to initialize rc. Also, let's return 0 directly when
succeeding.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-4-david@redhat.com