mlxsw: spectrum: Prevent RIF access outside of routing code

There are currently 5 users of mlxsw_sp_rif_find_by_dev() outside of the
routing code. Only one call site actually needs to dereference the
router interface (RIF). The rest merely need to know if a RIF exists for
the provided netdev.

Convert this call site to query the needed information directly from the
routing code instead of dereferencing the RIF.

This will later allow us to replace mlxsw_sp_rif_find_by_dev() with a
function that checks if a RIF exist.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>