ath9k: fix per-packet tx power configuration

Do not use ieee80211_vif pointer in ath_get_rate_txpower() since it has been
overwritten by setup_frame_info() and it will result in a corrupted tx power
configuration. Set per-packet tx power in setup_frame_info() according to
current vif tx power.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>