xpt_async is submitting a CCB, not finishing it up, so use xpt_action() instead
of xpt_done(). Add the missing XPT_ASYNC case to xpt_action_default. xpt_async
wants to use the side-effect of the xpt_done() routine to queue this to the
camisr thread so it can be done in that context. However, this breaks the
symmetry that you create a ccb and call xpt_action() for it to be
dispatched. Restore that symmetry by having it go through that path. As far as I
can tell, this is the only CCB that we create and call xpt_done() on directly.