audio passthrough: fix forcing line in when trrs and line in are plugged in
Apparently I missed this at camp as I lacked a trrs cable (rather I was too lazy to go to my tent and fetch one but shh).
If you have both a trrs cable on line out and a cable on line-in, it defaults to trrs and does not let you force line-in mode.
This MR changes it so that line-in is preferred as a source, and lets all modes be forced as desired.
I've also verified that after this change everything works as intended (sans #70 (closed) but that's not a part of this) soo I shouldn't need to tweak this further very much.
