ath11k: fix debugfs build failure

When CONFIG_ATH11K_DEBUGFS is disabled, but CONFIG_MAC80211_DEBUGFS
is turned on, the driver fails to build:

drivers/net/wireless/ath/ath11k/debugfs_sta.c: In function 'ath11k_dbg_sta_open_htt_peer_stats':
drivers/net/wireless/ath/ath11k/debugfs_sta.c:416:4: error: 'struct ath11k' has no member named 'debug'
  ar->debug.htt_stats.stats_req = stats_req;
    ^~

It appears that just using the former symbol is sufficient here,
adding a Kconfig dependency takes care of the corner cases.

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>