- Streamline detach logic in wlan drivers, so that

- Streamline detach logic in wlan drivers, so that
  freed memory cannot be used during detach.
- Remove all panic() calls from the urtw driver because
  panic() is not appropriate here.
- Remove redundant checks for device detached in
  device detach callbacks.
- Use DEVMETHOD_END to mark end of device methods.

MFC after:	2 weeks