s390x/css: Don't save orb in subchannel.

s390x/css: Don't save orb in subchannel.

Current css code saves the operation request block (orb) in the
subchannel structure for later consumption by the start function
handler. This might make sense for asynchronous execution of the
start function (which qemu doesn't support), but not in our case;
it would even be wrong since orb contains a reference to a local
variable in the base ssch handler.

Let's just pass the orb through the start function call chain for
ssch; for rsch, we can pass NULL as the backend function does not
use any information passed via the orb there.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>