mlxsw: spectrum_router: Extend the RIF struct

Currently, when a Subport RIF is configured, the LAG status and VLAN of
the underlying port are read from the port itself. This is problematic,
as we would like to have common code to configure all types of RIFs,
which aren't necessarily bound to a port.

Instead, embed the RIF in a struct specific to the Subport type, which
contains all the necessary information.

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