ath9k: Only process fft samples when ATH9K_DEBUGFS is enabled

The code can only be used when ATH9k_DEBUGFS is enabled an not when ATH_DEBUG
is activated. Still enabling it would cause build failures.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>