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>

drivers/net/wan/sealevel.c

@@ -241,6 +241,7 @@ static inline struct slvl_device *slvl_alloc(int iobase, int irq)
 		return NULL;
 
 	sv = d->priv;
+	d->ml_priv = sv;
 	sv->if_ptr = &sv->pppdev;
 	sv->pppdev.dev = d;
 	d->base_addr = iobase;