fpga: sim: Update struct enum initialization

In Vivado 2021.1, it is no longer allowed to use "default: 0" to
initialize an enum within a packed struct. So we must initalize enum
data members explicitly now.


Original-commit: 8007b6b4dff5b64d476b2e3ccb68a777c2f02c8b

lib/sim/rfnoc/axi_rate_change/axi_rate_change_tb.sv

@@ -153,7 +153,7 @@ module axi_rate_change_tb();
 
         while (words_left_to_send > 0) begin
           // Setup header
-          send_header = '{default:0};
+          send_header = '0;
           send_header.has_time = 1;
           send_header.timestamp = longint'(timestamp);
           send_header.eob = (words_left_to_send <= spp);