ath9k: fix BE/BK queue order

Hardware queues are ordered by priority. Use queue index 0 for BK, which
has lower priority than BE.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>