syncppp: Fix crashes.

The syncppp layer wants a mid-level netdev private pointer.

It was using netdev->priv but that only worked by accident,
and thus this scheme was broken when the device private
allocation strategy changed.

Add a proper mid-layer private pointer for uses like this,
update syncppp and all users, and remove the HDLC_PPP broken
tag from drivers/net/wan/Kconfig

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/syncppp.h b/include/net/syncppp.h
index 877efa4..e43f407 100644
--- a/include/net/syncppp.h
+++ b/include/net/syncppp.h
@@ -59,7 +59,7 @@
 
 static inline struct sppp *sppp_of(struct net_device *dev) 
 {
-	struct ppp_device **ppp = dev->priv;
+	struct ppp_device **ppp = dev->ml_priv;
 	BUG_ON((*ppp)->dev != dev);
 	return &(*ppp)->sppp;
 }