Flesh out a new HAL method to fetch the radar PHY error frame information.

For the AR5211/AR5212, this is apparently a one byte pulse duration
counter value. It is only coded up here for the AR5212 as I don't have
any AR5211-series hardware to test it on.

This information was extracted from the Madwifi DFS branch along with
some local additions.

Please note - all this does is extract out the radar event duration,
it in no way reflects the presence of a radar. Further code is needed
to take a set of radar events and filter them to extract out correct
radar pulse trains (and ignore other events.)

For further information, please see:

http://wiki.freebsd.org/dev/ath_hal%284%29/RadarDetection

This includes references to the relevant patents which describe what
is going on.

Obtained from: Madwifi