s390x: s390_cpu_get_phys_page_debug has to return -1

If translation fails, we have to return -1. For now, we
would simply return the value last stored to raddr (if any).

This way, reading invalid memory via gdb will return values, although it
shouldn't.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>