ath9k: DFS - consider ext_channel pulses only in HT40 mode

The chip reports radar pulses on extension channel
even if operating in HT20 mode. This patch adds a
sanity check for HT40 mode before it feeds pulses
on extension channel to the pattern detector.

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