mlxsw: spectrum_router: Update tunnel decap properties

Spectrum-2 requires to specify the egress RIF when setting tunnel decap
properties. Add a method for accessing the underlay RIF index and then use
it when setting decap properties.

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>