ath9k: declare required extra tx headroom

ath9k inserts padding between the 802.11 header and the data area (to
align it). Since it didn't declare this extra required headroom, this
led to some nasty issues like randomly dropped packets in some setups.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>