eth+ipv4prot: Fix protocol arbitration

Fixes #13.

trashernet/trashernet_eth.vhd

@@ -141,7 +141,10 @@ begin
 						end if;
 
 					when TXD =>
-						state <= IDLE when (not mac_in.tx_mac_data_en or not mac_out.tx_active);
+						if (not mac_in.tx_mac_data_en or not mac_out.tx_active) then
+							state <= IDLE;
+							sel <= SEL_ETH_I; -- To avoid arbitration errors, always select the highest priority one by default
+						end if;
 				end case;
 			end if;
 		end process arb;

trashernet/trashernet_ipv4prot.vhd

@@ -105,7 +105,10 @@ begin
 						end loop;
 
 					when TXD =>
-						state <= IDLE when (not ipv4_in.tx_en);
+						if not ipv4_in.tx_en then
+							state <= IDLE;
+							tx_sel <= ipv4_protocol_in'left; -- To avoid arbitration errors, always select the highest priority one by default
+						end if;
 				end case;
 			end if;
 		end process arb;