S2IO: Added a loadable parameter to enable or disable vlan stripping in frame.

- Added code to not to strip vlan tag when driver is in promiscuous mode
- Added module loadable parameter 'vlan_tag_strip" through which user can
  enable or disable vlan stripping irrespective of mode
  ( promiscuous or non-promiscuous ).

Signed-off-by: Sivakumar Subramani <sivakumar.subramani@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>