s390x/sclp: Add missing checks to SCLP handler

If the 51 most significant bits of the SCCB address are zero or equal to
the prefix, we should throw an specification exception, too.
Also moved the check for privileged mode to sclp_service_call() to have
all program checks in one place now.

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