Currently we create and destroy channels with the following consistent
pattern:
- chn_init() -> pcm_chn_add()
- pcm_chn_remove() -> chn_kill()
Instead of calling two separate functions, merge pcm_chn_add() with
chn_init(), and pcm_chn_remove() with chn_kill().
Another benefit of this change is that we avoid the confusion caused by
having pcm_chn_add(), as well as pcm_addchan().
Sponsored by: The FreeBSD Foundation
MFC after: 2 days