ath9k: fix BE/BK queue order

commit 78063d81d353e10cbdd279c490593113b8fdae1c upstream.

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

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>