ath9k: Fix compilation breakage

Since ath9k makes use of mac80211's debugfs hooks to
maintain station statistics, make ATH9K_DEBUGFS
select MAC80211_DEBUGFS. This fixes the issue reported by
Fengguang Wu:

drivers/net/wireless/ath/ath9k/debug.c: In function 'ath9k_sta_add_debugfs':
drivers/net/wireless/ath/ath9k/debug.c:1589:4: error: 'struct ath_node' has no member named 'node_stat'
drivers/net/wireless/ath/ath9k/debug.c: In function 'ath9k_sta_remove_debugfs':
drivers/net/wireless/ath/ath9k/debug.c:1599:19: error: 'struct ath_node' has no member named 'node_stat'

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>