mac80211: small rate control changes

This patch fixes mac80211 to not rely on the rate control
algorithm to update sta->tx_retry_failed and sta->tx_retry_count
(even if we don't currently use them), removes a number of
completely unused values we don't even show in debugfs and
changes the code in ieee80211_tx_status() to not look up the
sta_info repeatedly.

The only behaviour change here would be not calling the rate
control function rate_control_tx_status() when no sta_info is
found, but all rate control algorithms ignore such calls anyway.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>