If a host controller has been detached or reported a halt condition it

If a host controller has been detached or reported a halt condition it
should be considered as dead.  In such condition the stack no longer
submits I/O or schedules any USB task for its bus.

However we need an explore task to detach the root hub since only the
task thread is supposed to discover buses and attach/detach devices in
order to avoid races.

So reset the bus' dying flag when marking it as disconnecting in order
to let the task be scheduled.  This should be safe because a detached
or halted HC should not trigger any new interrupt.

Fix a panic when detaching USB PCMCIA cards reported by Tilo Stritzky.