s390/cio: use cssid for pgid generation

Obtain the real channel subsystem id and use that for the generation
of a unique path group id. Note that this change does not affect the
channel subsystem id as used in the user-visible naming of subchannels
and friends.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshik@linux.vnet.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>