mac80211: implement ieee80211_tx_rate_update to update rate

Current mac80211 has provision to update tx status through
ieee80211_tx_status() and ieee80211_tx_status_ext(). But
drivers like ath10k updates the tx status from the skb except
txrate, txrate will be updated from a different path, peer stats.

Using ieee80211_tx_status_ext() in two different paths
(one for the stats, one for the tx rate) would duplicate
the stats instead.

To avoid this stats duplication, ieee80211_tx_rate_update()
is implemented.

Signed-off-by: Anilkumar Kolli <akolli@codeaurora.org>
[minor commit message editing, use initializers in code]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>