mlxsw: spectrum_router: Support RIF without device

Spectrum-2 underlay RIF is merely an auxiliary RIF that points to the
virtual router used for encapsulated packets lookup. It exists only when
its overlay RIF exists but may be shared with other overlay RIFs.
Hence it is undesired to mark any device as related to it.

Therefore allow usage of NULL device when allocating RIF.

Signed-off-by: Nir Dotan <nird@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>