o leave IEEE80211_RADIOTAP_HDRLEN for portability to other systems but
correct comment about radiotap headers being padded to 64-bytes
(hasn't been true for many years)
o remove reference to IEEE80211_RADIOTAP_FCS; it was never used, instead
the flags are marked with IEEE80211_RADIOTAP_F_FCS to indicate whether
or not FCS is present
Might be better to just remove IEEE80211_RADIOTAP_HDRLEN so drivers
don't bogusly pad.
MFC after: 2 weeks