- In command RX intr, if the command number returned is 0, then hardware

- In command RX intr, if the command number returned is 0, then hardware
  will not work properly afterwards.  This usually happens after the whole
  box is power off and on.  Reinitializing the hardware in a tight loop
  several times does not work in this case.  If this happens, set a callout
  to reinitialize the hardware 1 seconds later.
- Stop various callouts in iwl2100_{init,stop}() before dispatching the
  real work out to device thread.
- Always stop all callouts, if IFF_UP is turned off in ifnet.if_flags.
- Add a flag to mark that the device is detaching, mainly to avoid possible
  stop/(re)init/restart ordering problem in device thread during detaching.