sound: Use bus_child_deleted methods to free ivars for children

Note that hdsp and hdspe were just leaking the ivars on detach
previously.

While here, use M_WAITOK to allocate ivars since attach routines are
sleepable. hdsp and hdspe were using M_NOWAIT without checking the
return value.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47366