usb-audio: New mixer control quirk for Digidesign Mbox

[Damien Zammit]

Hi all,

I just figured out how to toggle the input source between analog/spdif on the digidesign mbox.

The problem is, when I toggle the spdif source, the hardware auto-updates to spdif clock sync as well,
so I need to update the other mixer control to reflect the new hardware state or force an update
to revert the clock source, but I need to know the current state of the other mixer control.

How do I access a second mixer control from another one to synchronise the mixer states?

Also, how would I access the current sample rate from inside a mixer control?

Damien