ath9k: add DFS radar pulse processing

This initial DFS module provides basic functionality to deal
with radar pulses reported by the Atheros DFS HW pulse detector.

The reported data is evaluated and basic plausibility checks
are performed to filter false pulses. Passing radar pulses are
forwarded to pattern detectors which are not yet implemented.

(Some modifications to actually use ATH9K_DFS_DEBUGFS based on comments
from Julian Calaby <julian.calaby@gmail.com>. -- JWL)

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>