Mixer operation is interrupted on USB streaming events


Consider the situation where the mixer is conjured to, say, output a mix of anagoe inputs to an analogue inputs. 

The user would not expect this mix to be interfered with based on usb streaming events. This is currently not the case. 

I believe this regression in behaviour was added when adding 16bit or even DSD support. Likely exasperated with the low power work (added a stream stop event).