[SCSI] zfcp: Access ports and units with container_of in sysfs code

When accessing port and unit attributes, use container_of instead of
dev_get_drvdata. This eliminates some code checker warnings about
aliased access of data structures.

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>