add a FreeBSD_version check. It should work fine for compiling
on -HEAD, 9.x and 8.x.
* Conditionally compile the 11n options only when 11n is enabled.
The above changes allow the ath(4) driver to compile and run on
8.1-RELEASE (Hi old PC-BSD!) but with the 11n stuff disabled.
I've done a test against the net80211 and tools in 8.1-RELEASE.
The NIC used in testing is the AR2427 in an EEEPC.
Just to be clear - this change is to allow the -HEAD ath/hal/rate
code to run on 9.x _and_ 8.x with no source changes. However,
when running on earlier kernels, it should only be used for legacy
mode. (Don't define ATH_ENABLE_11N.)