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>