[S390] cio: css_register_subchannel race.

Asynchronous probe can release memory of a subchannel before
css_get_ssd_info is called. To fix this call css_get_ssd_info
before registering with driver core.

Signed-off-by: Stefan Bader <shbader@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>