emdtv(4): Fix issues in detach.

- Use config_detach_children, and do it up front, and handle failure
  (not relevant for yanking usb but relevant for drvctl which doesn't
  pass DETACH_FORCE).

- Fix teardown order: stop interrupts so we stop issuing new work,
  and _then_ wait for pending work to drain and destroy the
  workqueue.

- Omit needless empty  mutex_enter(lock); mutex_exit(lock)  dance
  which probably only appeared necessary because of the wrong
  teardown order.

(riastradh)