Add channel survey support to the AR5212 HAL.

The AR5212 series of MACs implement the same channel counters as the
later 11n chips - except, of course, the 11n specific counter (extension
channel busy.)

This allows users of these NICs to use 'athsurvey' to see how busy their
current channel is.

Tested:

* AR5212, AR2413 NICs, STA mode

Approved by:	re@ (gleb)