Enable/disable flow control

        Entry:
          AH = 0Fh
          AL = bit mask describing flow control requested
              0: xon/xoff on transmit (watch for xoff while sending)
              1: CTS/RTS (CTS on transmit/RTS on receive)
              2: reserved
              3: xon/xoff on receive (send xoff when buffer near full)
              4-7: all 1
          DX = port number