selftests: mlxsw: q_in_q_veto: Disable IPv6 autogen on bridges

In a future patch, mlxsw will start adding RIFs to uppers of front panel
port netdevices, if they have an IP address.

The swp enslavement to the 802.1ad bridge is not allowed, because RIFs are
not allowed to be created for 802.1ad bridges, but the address indicates
one needs to be created. Thus the veto selftests fail already during the
port enslavement. Then the attempt to create a VLAN on top of the same
bridge is not vetoed, because the bridge is not related to mlxsw, and the
selftest fails.

Fix by disabling automatic IPv6 address generation for the bridges in this
selftest, thus exempting them from the mlxsw router attention.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>