s390x: smp: Dirty fpc before initial reset test

Let's dirty the fpc, before we test if the initial reset sets it to 0.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200429143518.1360468-3-frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>