ALSA: usb-audio: Add mixer quirk for RME Digiface USB

Implement sync, output format, and input status mixer controls, to allow
the interface to be used as a straight ADAT/SPDIF (+ Headphones) I/O
interface.

This does not implement the matrix mixer, output gain controls, or input
level meter feedback. The full mixer interface is only really usable
using a dedicated userspace control app (there are too many mixer nodes
for alsamixer to be usable), so for now we leave it up to userspace to
directly control these features using raw USB control messages. This is
similar to how it's done with some FireWire interfaces (ffado-mixer).

Signed-off-by: Asahi Lina <lina@asahilina.net>
Link: https://patch.msgid.link/20240903-rme-digiface-v2-2-71b06c912e97@asahilina.net


Signed-off-by: Takashi Iwai <tiwai@suse.de>