S2io: Support for vlan_rx_kill_vid entry point

- Resubmit #3
- Added s2io_vlan_rx_kill_vid entry point function for unregistering vlan.
- Fix to aggregate vlan packets. IP offset is incremented by
  4 bytes if the packet contains vlan header.

Signed-off-by: Surjit Reang <surjit.reang@neterion.com>
Signed-off-by: Ramkrishna Vepa <ram.vepa@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>