Update MyTransportPJON.cpp (mysensors#1526)

Enabling to define different value of PJON_POLLING_DURATION

Author: gryzli133

hal/transport/PJON/MyTransportPJON.cpp

@@ -22,7 +22,10 @@
  */
 
 // Set a higher polling duration if the device is executing long tasks
+
+#ifndef PJON_POLLING_DURATION
 #define PJON_POLLING_DURATION 1000
+#endif
 
 PJONSoftwareBitBang bus;