[SCSI] zfcp: prevent fc_remote_port_delete calls for unregistered rport

In case of an adapter reopen all rports have to be deleted from the
environment. This should only happen for already registered rports
otherwise fc_remote_port_delete is called with a NULL pointer.

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