ath9k: add per-vif TX power capability

Configure the HW with highest TX power among all vif when HW TPC has been
enabled in order to add support to per-vif TX power capability. Use lowest
configured power among all interfaces when TPC is disabled

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