mlxsw: spectrum_router: Add mlxsw_sp_rif_fid()

In order to allow querying of the VID for which a RIF was created, add
a new function that returns a FID for a given RIF.

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