Use radiotap parser from radiotap.org

Fixes a problem with multiple signal values of multiple antennas (chains) as
reported by recent mac80211 in 802.11N mode (ath9k) and in general is better
than maintaining my own limited homebrewn version of a radiotap parser.
Avoid redefining le64toh etc if already defined

Fixes a warning when compiling for OpenWRT