mlxsw: spectrum: Export function to check if RIF exists

After the previous patch, all the callers of mlxsw_sp_rif_find_by_dev()
outside of the routing code use it to understand if a RIF exists for the
passed netdev.

Therefore, export a function to check if a RIF exists and make
mlxsw_sp_rif_find_by_dev() internal to the routing code.

This will later allow us to more easily introduce the router lock which
will also protect the RIFs.

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