[S390] cio: merge init calls

Define initialization sequence of css and ccw bus init calls by merging
them into a single init call. Also introduce channel_subsystem_init_sync
to wait for the initialization of devices to finish.

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