ath9k: optimize ath_tx_rc_status usage

The only flag that needs to be set when ath_tx_rc_status is called with
rc_update == false is the IEEE80211_TX_STAT_TX_FILTERED flag. All other
data is ignored in that case.
This flag can be set from ath_tx_complete_buf instead, so that we can
drop a few redundant calls to ath_tx_rc_status and remove the rc_update
function parameter

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>