RDMA/cxgb4: Suppress gcc 7 fall-through complaints

Avoid that gcc 7 reports the following warning when building with W=1:

warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/cxgb4/cm.c

@@ -3638,6 +3638,7 @@ static void active_ofld_conn_reply(struct c4iw_dev *dev, struct sk_buff *skb,
 			send_fw_act_open_req(ep, atid);
 			return;
 		}
+		/* fall through */
 	case FW_EADDRINUSE:
 		set_bit(ACT_RETRY_INUSE, &ep->com.history);
 		if (ep->retry_count++ < ACT_OPEN_RETRY_COUNT) {