Soft jack injection for USB audio?

[Kai-Heng Feng]

I have the need to use soft jack injection to debug userpsace, but
currently USB audio doesn't have that ability.

The problem I am facing is that the USB audio doesn't use snd_jack_*
to control the jack, so we can add the support for USB audio
separately.

It's not hard to add the support if we open code soft injection in USB
audio's build_connector_control(), but would it be possible to use
snd_jack_* in USB audio?

Kai-Heng