ixgbe: fix wrong offset to fc_frame_header in ixgbe_fcoe_ddp

Make sure we point to the right offset of the fc_frame_header when
VLAN header exists and HW has VLAN stripping disabled.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>