ath9k: DFS - add pulse chirp detection for FCC

FCC long pulse radar (type 5) requires pulses to be
checked for chirping. This patch implements chirp
detection based on the FFT data provided for long
pulses.

A chirp is detected when a set of criteria defined
by FCC pulse characteristics is met, including
* have at least 4 FFT samples
* max_bin index moves equidistantly between samples
* the gradient is within defined range

The chirp detection has been tested with reference
radar generating devices and proved to work reliably.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>