mlxsw: spectrum_router: Fix error path in mlxsw_sp_router_init

When neigh_init fails, we have to do proper cleanup including
router_fini call.

Fixes: 6cf3c971

 ("mlxsw: spectrum_router: Add private neigh table")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>