Revert "sound: Merge chn_intr() with chn_intr_locked()"

It turns out that snd_uaudio(4) uses sound(4)'s channel lock for its USB
transfer callbacks. I will try to address this at some point, because
this is layering violation, but for now we need to revert the commit, as
it causes a lock recursion panic with USB audio devices.

This reverts commit e254ef87a30bfcaabc6e4d8e0ecf05f6949a4f06.