ath9k: make CONFIG_ATH9K_DFS_CERTIFIED depend on CFG80211_CERTIFICATION_ONUS

Turns out every most standard Linux distributions enable
CONFIG_EXPERT, so use the shiny new CFG80211_CERTIFICATION_ONUS
which is meant by design to not be enabled by all Linux
distributions.

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