snd_uaudio: recognize hardware sidetone as a monitorHEADmain

The Logitech H390, for instance, has the following interface layout:

~~
7 INPUT              34 INPUT            10 INPUT
  Mic (0x201)          Mic (0x201)         USB Stream (0x101)
   |                    |                     |
   v                    v                     |
 19 FEATURE           35 FEATURE              |
   |                    |                     |
   v                    v                     |
 25 EXTENSION           +------> 36 MIXER <---+
   |                              |
   v                              v
 13 OUTPUT                     22 FEATURE
 USB Stream (0x101)               |
                                  v
                               16 OUTPUT
                               Speaker (0x301)
~~

The 7->13 path on the left is a typical microphone-in configuration,
while the right side is a little more complicated. The 34 -> 35 -> 36
leg is describing a hardware sidetone control, while the other is a
standard audio-out configuration.

During feature unit evaluation, we need to pick up the scenario of node
35 above, which is directly wiring the microphone to the speaker.  Right
now we'll likely tie it to the pcm/vol levels and this unit will emit
a very prompt feedback screech, but it's really shaped more like a
MONITOR control.

This avoids mishandling feature unit 22 because that's evaluated in one
of the other cases: one of the inputs is the USB stream, so it's
wired up as a PCM.

One note on this headset: the presence of mixer 36 currently breaks the
`vol` control, leaving only `pcm` to control the volume.  Given that it
has both Mic and USB input, I suspect we get a 1:1 cluster configuration
for the Mic input but something more complicated for the USB input that
we end up ignoring.  Thus, "vol" might technically control the monitor
volume but isn't wired up to the USB input cluster.  I have not had a
chance to confirm this, yet.

PR:		291424
Reviewed by:	christos
Differential Revision:	https://reviews.freebsd.org/D58779

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