KVM: s390: Use helper function to set CC in SIGP handler

We've got a helper function for setting the condition code now,
so let's use it in the SIGP handler, too.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>