mac80211: report some VHT radiotap infos for tx status

mac80211: report some VHT radiotap infos for tx status

The radiotap VHT info is 12 bytes (required to be aligned on 2) :
u16 known - IEEE80211_RADIOTAP_VHT_KNOWN_*
u8 flags - IEEE80211_RADIOTAP_VHT_FLAG_*
u8 bandwidth
u8 mcs_nss[4]
u8 coding
u8 group_id
u16 partial_aid

ATM mac80211 can handle IEEE80211_RADIOTAP_VHT_KNOWN_{GI,BANDWIDTH} and
mcs_nss[0] (i.e single user) in simple cases.
This is more a placeholder to let sniffers give more clues for VHT,
since we don't have yet the proper infrastructure/conventions
in mac80211 for complete feedback (e.g consider dynamic BW).

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>