Calculate checksum for TCP data also

Right now we calculate the checksum for the establish SYN/ACK messages, but we do not calculate the checksum for the data packets, as it involves calculating the checksum before we transmit the data. To do this we will need a fifo that is at least one packet size (2048) to run the data through to calculate the checksum, then we can add this checksum to the header checksum to get the total checksum.