s390/cmf: fix kerneldoc

Make sure we use proper Return sections, and make the output
for cmf_enable() less odd.

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