Update axis_xgmii_tx_64.v

Fix for alexforencich#226

rtl/axis_xgmii_tx_64.v

@@ -171,7 +171,7 @@ reg [4+16-1:0] ts_inc_reg = 0;
 reg [DATA_WIDTH-1:0] xgmii_txd_reg = {CTRL_WIDTH{XGMII_IDLE}}, xgmii_txd_next;
 reg [CTRL_WIDTH-1:0] xgmii_txc_reg = {CTRL_WIDTH{1'b1}}, xgmii_txc_next;
 
-reg start_packet_reg = 2'b00;
+reg [1:0] start_packet_reg = 2'b00;
 reg error_underflow_reg = 1'b0, error_underflow_next;
 
 assign s_axis_tready = s_axis_tready_reg;