cfg80211/mac80211: refactor cfg80211_chandef_dfs_required()

Some interface types don't require DFS (such as STATION, P2P_CLIENT
etc).  In order to centralize these decisions, make
cfg80211_chandef_dfs_required() take the iftype into consideration.

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