[SCSI] zfcp: Fix retry after failed "open port" erp action

Trying to enqueue a port erp action from the port erp strategy will
fail in zfcp_erp_required_act. To try the same action again, return
ZFCP_ERP_FAILED.

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>