IQ tool: Always choose correct sampling rate

Reselect file before starting playback. Fixes incorrect sample rate
when playback is started, stopped, devices switched, dsp started,
stopped and then started playback of the same IQ file.

src/qtgui/iq_tool.cpp

@@ -127,6 +127,7 @@ void CIqTool::on_playButton_clicked(bool checked)
  {
  ui->listWidget->setEnabled(false);
  ui->recButton->setEnabled(false);
+ on_listWidget_currentTextChanged(current_file);
  emit startPlayback(recdir->absoluteFilePath(current_file),
  (float)sample_rate, center_freq);
  }