Add a common command structure #537
Description
Currently the commands that are forwarded from EP0 or EP buffer to Audiohub have a variable structure (one command word followed by upto 2 words of data). This means that even the stages that are just forwarding the command, like the mixer, need to be aware of the command type to receive the forward the required amount of data.
If all commands had a common struct (command and 2 words of data for every command for instance), mixer could be agnostic of the commands it is forwarding.
I attempted this change here (1b0027c) but it ended up being more complicated than I anticipated. The complicated case was the i2s only build which doesnt have the decoupler, and EP0 is connected directly to mixer, requiring EP0 to also we aware of the command structure.