checkPort

checkPort    helper function to check if a port is a valid transmission port
   portnum = checkPort(tx,ports)
   inputs:
       tx: [int] port number of the Transmitter
       ports: mapping dictionary,
           possible keys: ['Tx1','Tx2','Rx1A','Rx1B','Rx2A','Rx2B']
           possible values: [1, 2, 3, 4, 5, 6]
   output:
       portnum: [int] returns 1 if port is 'Tx1' and 2 if port is 'Tx2'