Enable AC3 encoding only if "optical" device type is selected

We really don't want to trigger AC3 encoding just because the user
selected stereo channels and AC3 passthrough. In general, we want to
avoid AC3 encoding except if the user explicitly enables it, which is by
using the "optical" device type.

src/player/PlayerComponent.cpp

@@ -942,7 +942,7 @@ void PlayerComponent::setAudioConfiguration()
   // if we see user requests for it.
   //
   m_doAc3Transcoding = false;
-  if (layout == "2.0" &&
+  if (deviceType == AUDIO_DEVICE_TYPE_SPDIF &&
       SettingsComponent::Get().value(SETTINGS_SECTION_AUDIO, "passthrough.ac3").toBool())
   {
     mpv::qt::command(m_mpv, QStringList() << "af" << "add" << "@ac3:lavcac3enc");