Fix the calculation of remaining header length in TSO
over vlan device case. This was inadvertently missed
out in patch 28fa548fcdd4e0f1c454886 ("netxen: add vlan
tx acceleration support").
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com> Signed-off-by: David S. Miller <davem@davemloft.net>