mac80211: add xmit rate to struct ieee80211_tx_status

Right now struct ieee80211_tx_rate cannot hold HE rates. Lets use
struct ieee80211_tx_status instead. This will also make the code
future-proof for when we have EHT.

Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190714154419.11854-2-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>