s390/chpid: make headers usable (again)

Add back a hunk from "4dcc2a4d

 s390/chsc: make headers usable"
which was lost during the merge of the UAPI patch.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>