ath9k: Fix regression in 'xmit' debugfs file

Commit "ath9k: Fix the 'xmit' debugfs file" changed the
the array size of ath_stats.txstats to IEEE80211_NUM_ACS,
which is wrong because the HW queue number is used to
update the statistics. Revert back to using ATH9K_NUM_TX_QUEUES.

Reported-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>