cfg80211: allow restricting supported dfs regions

At the moment, the ath9k/ath10k DFS module only supports detecting ETSI
radar patterns.
Add a bitmap in the interface combinations, indicating which DFS regions
are supported by the detector. If unset, support for all regions is
assumed.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>