Flesh out the radar detection related operations for the ath driver.

This is in no way a complete DFS/radar detection implementation!
It merely creates an abstracted interface which allows for future
development of the DFS radar detection code.

Note: Net80211 already handles the bulk of the DFS machinery,
all we need to do here is figure out that a radar event has occured
and inform it as such. It then drives the DFS state engine for us.

The "null" DFS radar detection module is included by default;
it doesn't require a device line.

This commit:

• Adds a simple abstracted layer for radar detection state - sys/dev/ath/ath_dfs/;
• Implements a null DFS module which doesn't do anything; (ie, implements the exact behaviour at the moment);
• Adds hooks to the ath driver to process received radar events and gives the DFS module a chance to determine whether a radar has been detected.

Obtained from: Atheros