cfg80211: enable use of non-cleared DFS channels for DFS offload

Currently channel switch/start_ap to DFS channel cannot be done to
non-CAC-cleared channel even if DFS offload if enabled.
Make non-cleared DFS channels available if DFS offload is enabled.
CAC will be started by HW after channel change, start_ap call, etc.

Signed-off-by: Dmitry Lebed <dlebed@quantenna.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>