mac80211: add radiotap flag and handling for 5/10 MHz

Wireshark already defines radiotap channel flags for 5 and 10 MHz, so
just use them in Linux radiotap too. Furthermore, add rx status flags to
allow drivers to report when they received data on 5 or 10 MHz channels.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>