setSpeed() - eliminate spurious -v media log line if Media Monitor di…

…sabled.

If user has the Media Monitor disabled, this happened:
2012-03-07 12:22:34.376477 I  MythCDROMDarwin::setSpeed() - CD/DVD Speed Set to 1
2012-03-07 12:22:34.376616 I  MediaMonitor::setSpeed(//dev/rdisk1) - Cannot find/create CDROM?

mythtv/libs/libmyth/mythmediamonitor.cpp

@@ -97,6 +97,7 @@ void MediaMonitor::SetCDSpeed(const char *device, int speed)
     {
         cd->setSpeed(device, speed);
         delete cd;
+        return;
     }
 
     LOG(VB_MEDIA, LOG_INFO,