[SCSI] zfcp: Wait for port scan to complete when setting adapter online

Attaching a unit immediately after setting the adapter online should
be possible. The problem right now is that the port_scan runs from a
workqueue and has not finished when the set_online call returns and
the sysfs structures for the ports are not available yet. Fix that by
waiting for the port scan to complete.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>