Fix a race condition in detach path of Ethernet devices. Most current

Fix a race condition in detach path of Ethernet devices. Most current
drivers do something like this:

XX_stop();
ether_ifdetach();

/* continue and assume device is dead now */

This doesn't work in many cases if a BPF user is left and IFF_PROMISC is
active, because bpfdetach does a reset of the timer for many drivers.
The fix is to move the call to if_down into ether_ifdetach, which gives
the expected behaviour.