s390: fix comment for scsw_cmd_is_valid_sctl

This function is checking for the suspend control, not the function
control.

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>