Use a local STAILQ for unlocked done CCB processing in ahci direct mode

Previously it was possible for issues e.g. use after free, to result
from processing the done queue while not holding the channel lock.

While this should never happen in practice, unexpected code flows
which result in two threads processing from the same queue may
be possible.

We now use a local STAILQ to prevent this ever being an issue.

Sponsored by:	Multiplay