wan: dlci: Remove unused netdev_priv pointer

Remove occurrences of unused pointer to network device private data in
functions dlci_header() and dlci_receive().

Detected by Coverity: CID 139844, CID 139845.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: David S. Miller <davem@davemloft.net>