qeth: Exploit Connection Isolation

Isolate data connection to a shared OSA card against other data
connections to the same OSA card. Connectivity between isolated
data connections sharing the same OSA card is therefore possible only
through external network gear (e.g. a router).

Signed-off-by: Einar Lueck <elelueck@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>