ath9k: add DFS support for extension channel

In HT40 modes, pulse events on primary and extension
channel are processed individually. If valid, a pulse
event will be fed into the detector
* for primary frequency, or
* for extension frequency (+/-20MHz based on HT40-mode)
* or both

With that, a 40MHz radar will result in two individual
radar events.

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