s390x: make smp_cpu_setup() return 0 on success

Properly return "0" on success so callers can check if the setup was
successful.

The return value is yet unused, which is why this wasn't noticed so far.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20211202123553.96412-2-david@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>