mlxsw: spectrum_router: Use eXtended mezzanine to offload IPv4 router

In case the eXtended mezzanine is present on the system, use it for IPv4
router offload.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>