cfg80211: remove BUG_ON usage

These really can't trigger unless somebody messes up the code,
but don't make debugging it needlessly complicated, WARN and
return instead of BUG_ON().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/core.c

@@ -690,7 +690,7 @@ void wiphy_unregister(struct wiphy *wiphy)
 	rtnl_lock();
 	rdev->wiphy.registered = false;
 
-	BUG_ON(!list_empty(&rdev->wdev_list));
+	WARN_ON(!list_empty(&rdev->wdev_list));
 
 	/*
 	 * First remove the hardware from everywhere, this makes