Finish the transmission at the end of a correct packet

This patch restores the original behaviour of Cooja when
the transmitted packet is correct (which is true in a vast majority
of cases).

In case of a wrong outgoing packet (wrong length, wrong preamble)
the transmission will end when the radio changes its state (which
should always happen after transmitting a packet).

Benchmarks with RPL (33 runs, 50 nodes, 3 hours of simulated time each)
yield the same results (PDR, delay, number of transmitted packets)
as with the unmodified Cooja.

tools/cooja/apps/mspsim/src/org/contikios/cooja/mspmote/interfaces/Msp802154Radio.java

@@ -146,7 +146,7 @@ public class Msp802154Radio extends Radio implements CustomDataRadio {
             setChanged();
             notifyObservers();
           }
-          isSynchronized = false;
+          finishTransmission();
         }
       }
     }); /* addRFListener */