mlxsw: spectrum: Allow event handlers to check unowned bridges

Currently the bridge-related handlers bail out when the event is related to
a netdevice that is not an upper of one of the front-panel ports. In order
to allow enslavement of front-panel ports to bridges that already have
uppers, it will be necessary to replay CHANGEUPPER events to validate that
the configuration is offloadable. In order for the replay to be effective,
it must be possible to ignore unsupported configuration in the context of
an actual notifier event, but to still "veto" these configurations when the
validation is performed.

To that end, introduce two parameters to a number of handlers: mlxsw_sp,
because it will not be possible to deduce that from the netdevice lowers;
and process_foreign to indicate whether netdevices that are not front panel
uppers should be validated.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>