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.