qeth: Support VEPA mode

The existing port isolation mode 'forward' will now verify that the adjacent
switch port supports the required reflective relay (RR) mode. This patch adds
the required error handling for the cases where enabling port isolation mode
'forward' can now fail.
Furthermore, once established, we never fall back from one of the port
isolation modes to a non-isolated mode without further user-interaction.
This includes cases where the isolation mode was enabled successfully, but
ceases to work e.g. due to configuration changes at the switch port.
Finally, configuring an isolation mode with the device being offline
will make onlining the device fail permanently upon errors encountered until
either errors are resolved or the isolation mode is changed by the user to a
different mode.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Reviewed-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>