mlxsw: spectrum_router: Prepare for nexthops with trap action

Nexthops that need to be programmed with a trap action might not have a
valid router interface (RIF) associated with them. Therefore, use the
loopback RIF created during initialization to program them to the
device.

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