fix(SoftwareSerial): correct TX to RX switch when using half-duplex

Author: dominikberse

Diff for: libraries/SoftwareSerial/src/SoftwareSerial.cpp

@@ -157,6 +157,8 @@ bool SoftwareSerial::listen()
     active_listener = this;
     if (!_half_duplex) {
       active_in = this;
+    } else if (!active_out) {
+      setRXTX(true);
     }
     return true;
   }