ath9k: declare required extra tx headroom

ath9k: declare required extra tx headroom

Orabug: 22623801

commit 029cd0370241641eb70235d205aa0b90c84dce44 upstream.

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.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 1b4fe74c9f9af831cef9b9ec27dfdb22802838ef)
Signed-off-by: Dan Duval <dan.duval@oracle.com>