netxen: fix vlan tso case

Fix the calculation of remaining header length in TSO
over vlan device case. This was inadvertently missed
out in patch 028afe71 ("netxen: add vlan
tx acceleration support").

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>