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/drivers/net/wan/hdlc_ppp.c b/drivers/net/wan/hdlc_ppp.c
index 10396d9..0030833 100644
--- a/drivers/net/wan/hdlc_ppp.c
+++ b/drivers/net/wan/hdlc_ppp.c
@@ -45,7 +45,7 @@
 	int (*old_ioctl)(struct net_device *, struct ifreq *, int);
 	int result;
 
-	dev->priv = &state(hdlc)->syncppp_ptr;
+	dev->ml_priv = &state(hdlc)->syncppp_ptr;
 	state(hdlc)->syncppp_ptr = &state(hdlc)->pppdev;
 	state(hdlc)->pppdev.dev = dev;