cfg80211/mac80211: move more combination checks to mac80211

Get rid of the cfg80211_can_add_interface() and
cfg80211_can_change_interface() functions by moving that functionality
to mac80211.  With this patch all interface combination checks are now
out of cfg80211 (except for the channel switch case which will be
addressed in a future commit).

Additionally, modify the ieee80211_check_combinations() function so
that an undefined chandef can be passed, in order to use it before a
channel is defined.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/iface.c b/net/mac80211/iface.c
index 4826c8f..ad5badd 100644
--- a/net/mac80211/iface.c
+++ b/net/mac80211/iface.c
@@ -250,6 +250,7 @@
 {
 	struct ieee80211_local *local = sdata->local;
 	struct ieee80211_sub_if_data *nsdata;
+	int ret;
 
 	ASSERT_RTNL();
 
@@ -300,7 +301,10 @@
 		}
 	}
 
-	return 0;
+	mutex_lock(&local->chanctx_mtx);
+	ret = ieee80211_check_combinations(sdata, NULL, 0, 0);
+	mutex_unlock(&local->chanctx_mtx);
+	return ret;
 }
 
 static int ieee80211_check_queues(struct ieee80211_sub_if_data *sdata,