ath9k: non-functional: calculate DFS pulse interval per-wiphy

Systems with multiple DFS channel detectors need to track
timestamp of previous pulse per instance to display the
correct pulse interval.

Since the interval value is used for debug printing only,
this is a non-functional modification.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>