s390x: Give name to return value of tprot()

Improve readability by making the return value of tprot() an enum.

No functional change intended.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>