mixer: Fix default_unit switching with mixers that have no devices

Apparently it's possible for a mixer to have no devices:

$ mixer -f /dev/mixer2
pcm2:mixer: <USB audio> at ? kld snd_uaudio (rec)
$

If this is the default sound device, an attempt to change the default
unit using mixer -d fails with a segfault because mod_dunit is called
with a NULL device pointer, which is dereferenced to get the parent
mixer.

ctl_dunit seems to be a dummy, i.e., we don't actually need it and can
simply pass the mixer to mod_dunit() directly. This patch removes that
structure and associated indirection to fix the crash.

Reviewed by: christos, hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38060