As Steffen stated this is dependant on your firmware version. I believe this is a more recent addition.
The configuration item below should be what you are looking for. Include it in your master/shared cfg file. Attached is what the xml would look like but again this should be included in your master/shared cfg file not alone.
feature.usb.device.hostOs
Specify the operating system of the computer you are connecting by USB when using RealPresence Trio as an audio output device.
Windows (default) - The computer connected by USB to the RealPresence Trio uses a Windows operating system.
Other - The operating system of the computer connected via USB to the RealPresence Trio system is other than Windows or Mac.
Mac - The computer connected by USB to the RealPresence Trio uses a Mac operating system.
Confirm - The user is prompted the computer's operating system each time a USB cable is used to connect the RealPresence Trio 8800 system.
Good luck!