mac80211: add struct ieee80211_tx_status support to ieee80211_add_tx_radiotap_header

Add support to ieee80211_add_tx_radiotap_header() for handling rates
reported via ieee80211_tx_status. This allows us to also report HE rates.

Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190714154419.11854-4-john@phrozen.org
[remove text about 60 GHz, mac80211 doesn't support it, fix endianness issue]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>