[SCSI] zfcp: Call get_device on port before calling put_device

zfcp_unit_release calls put_device on the port. Ensure that get_device
has been called before possibly triggering the release function
through put_device or device_unregister.

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>