s2io: Fix enabling VLAN tag stripping at driver initialization

VLAN doesn't work except if you'd opened the interface in promiscuous
mode before.  This happens because VLAN tag stripping is not correctly
marked as enabled at device startup

Also, the vlan_strip_flag field was moved to the private network
structure.

Signed-off-by: Breno Leitao <leitao@linux.vnet.ibm.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>