selftests: forwarding: router_bridge: Use port MAC for bridge address

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

At the time that the front panel port is enslaved to the bridge, the bridge
MAC address does not have the same prefix as other interfaces in the
system. On Nvidia Spectrum-1 machines all the RIFs have to have the same
38-bit MAC address prefix. Since the bridge does not obey this limitation,
the RIF cannot be created, and the enslavement attempt is vetoed on the
grounds of the configuration not being offloadable.

The bridge eventually inherits MAC address from its first member, after the
enslavement is acked. A number of (mainly VXLAN) selftests already work
around the problem by setting the MAC address to whatever it will
eventually be anyway. Do the same here.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>