[SCSI] zfcp: Reorder registration of initial SCSI device

Make sure that the rport registration did complete and then register
SCSI device directly. Otherwise the unit_enqueue would race with the
call to zfcp_scsi_queue_unit_register.

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