snd_uaudio: Do not touch mixer_lock when there is no mixer

[christos]

Summary

Using the mixer lock unconditionally panics because the lock is
uninitialized on devices with no audio interfaces, for instance
MIDI-only keyboards.

Fixes: 9a00e0b8ca56 ("snd_uaudio: Do not use snd_mixer->lock as mixer_lock")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

[christos]

@markj @cperciva This affects stable/15 as well. I think it should go into releng/15.2 once MFC'd.

[markj]

sys/dev/sound/usb/uaudio.c

1153

This sentence is missing something, a device with no audio interfaces will do what?

[christos]

sys/dev/sound/usb/uaudio.c

1153

There is a typo. It was meant to say "... MIDI-only keyboard, will end up ...".