ath9k: Enable DFS only when ATH9K_DFS_CERTIFIED

Add DFS interface combination only when
CONFIG_ATH9K_DFS_CERTIFIED is set. In other case
user can run CAC/beaconing without proper handling
of pulse events (without radar detection activated).

Reported-by: Cedric Voncken <cedric.voncken@acksys.fr>
Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>