ath9k_hw: add DFS testing check

In order to enable DFS upstream we want to be sure
DFS has been tested for each chipset. Push for public
documentation of the requirements we want in place and
allow for enabling each chipset through a single upstream
commit.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>