DFS parameters fetched from the HAL.
Check whether the specific chipset supports RADAR reporting before
enabling DFS; or some of the (unset) DFS methods may fail.
Tested:
* AR5210 (correctly didn't enable radar PHY reporting)
* AR5212 (correctly enabled radar PHY reporting w/ the correct default
parameters.)
TODO:
* Now that I have this capability check in place, I could remove the
(empty) DFS methods from AR5210/AR5211.
* Test on AR5416, AR9160, AR9280.
PR: kern/170904