Transceiver52M: Set priority on downlink socket thread

Clock indications passed up to GSM core originate on the transciever
downlink side. Set priority to keep the flow of clock updates
consistent.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

Transceiver52M/Transceiver.cpp

@@ -852,6 +852,8 @@ void *TxUpperLoopAdapter(TransceiverChannel *chan)
 
   delete chan;
 
+  trx->setPriority(0.40);
+
   while (1) {
     bool stale = false;
     // Flush the UDP packets until a successful transfer.