ath9k: dfs: remove accidental use of stack VLA

In preparation to enabling -Wvla, remove accidental use of stack VLA.

This avoids an accidental stack VLA (since the compiler thinks
the value of FFT_NUM_SAMPLES can change, even when marked
"const"). This just replaces it with a #define.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath9k/dfs.c

@@ -41,7 +41,7 @@ static const int BIN_DELTA_MAX		= 10;
 
 /* we need at least 3 deltas / 4 samples for a reliable chirp detection */
 #define NUM_DIFFS 3
-static const int FFT_NUM_SAMPLES	= (NUM_DIFFS + 1);
+#define FFT_NUM_SAMPLES		(NUM_DIFFS + 1)
 
 /* Threshold for difference of delta peaks */
 static const int MAX_DIFF		= 2;
@@ -114,7 +114,7 @@ static bool ath9k_check_chirping(struct ath_softc *sc, u8 *data,
 
 		ath_dbg(common, DFS, "HT40: datalen=%d, num_fft_packets=%d\n",
 			datalen, num_fft_packets);
-		if (num_fft_packets < (FFT_NUM_SAMPLES)) {
+		if (num_fft_packets < FFT_NUM_SAMPLES) {
 			ath_dbg(common, DFS, "not enough packets for chirp\n");
 			return false;
 		}
@@ -136,7 +136,7 @@ static bool ath9k_check_chirping(struct ath_softc *sc, u8 *data,
 			return false;
 		ath_dbg(common, DFS, "HT20: datalen=%d, num_fft_packets=%d\n",
 			datalen, num_fft_packets);
-		if (num_fft_packets < (FFT_NUM_SAMPLES)) {
+		if (num_fft_packets < FFT_NUM_SAMPLES) {
 			ath_dbg(common, DFS, "not enough packets for chirp\n");
 			return false;
 		}