MFp4:

MFp4:

o don't grab the mutex at the top of ath_detach; it does nothing
  useful
o deal with entry to ath_ioctl during detach to disable promiscuous
  mode as a result of calling bpfdetach2: cannot call ath_init when
  the device is marked invalid as the code isn't prepared to deal
  with it (in particular by that time the hal reference may have
  been yanked)