nl80211: do a struct assignment to radar_chandef instead of memcpy()

We are copying one entire structure to another of the same type in
nl80211_notify_radar_detection, so it's simpler and safer to do a
struct assignment instead of memcpy().

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>