ath9k: dynack: Demote non-compliant function header

ath9k: dynack: Demote non-compliant function header

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/ath/ath9k/dynack.c:52: warning: Function parameter or member 'old' not described in 'ath_dynack_ewma'
 drivers/net/wireless/ath/ath9k/dynack.c:52: warning: Function parameter or member 'new' not described in 'ath_dynack_ewma'

Cc: QCA ath9k Development <ath9k-devel@qca.qualcomm.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201102112410.1049272-40-lee.jones@linaro.org