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/cfg.c b/net/mac80211/cfg.c
index 8bf94ea..23d6011 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -109,6 +109,15 @@
 static int ieee80211_start_p2p_device(struct wiphy *wiphy,
 				      struct wireless_dev *wdev)
 {
+	struct ieee80211_sub_if_data *sdata = IEEE80211_WDEV_TO_SUB_IF(wdev);
+	int ret;
+
+	mutex_lock(&sdata->local->chanctx_mtx);
+	ret = ieee80211_check_combinations(sdata, NULL, 0, 0);
+	mutex_unlock(&sdata->local->chanctx_mtx);
+	if (ret < 0)
+		return ret;
+
 	return ieee80211_do_open(wdev, true);
 }