[infiniband] Poll event queues only for devices that are open

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/infiniband.c

 static void ib_step ( struct process *process __unused ) {
 	struct ib_device *ibdev;
 
-	for_each_ibdev ( ibdev )
+	list_for_each_entry ( ibdev, &open_ib_devices, open_list )
 		ib_poll_eq ( ibdev );
 }