[WAN] hdlc_cisco: Fix regression introduced by skb->tail changes.

The following commit breaks cisco mode with my WAN drivers:
author	David S. Miller <davem@davemloft.net>
	Tue, 28 Jun 2005 22:25:31 +0000 (15:25 -0700)
commit	689be439



"[NET]: Remove gratuitous use of skb->tail in network drivers."

The following patch fixes it - please apply (cisco_hard_header does
skb_push(4 bytes)).

Signed-off-by: David S. Miller <davem@davemloft.net>