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>