mlxsw: spectrum_router: Join RIFs of LAG upper VLANs

In the following patches, the requirement that ports be only enslaved to
masters without uppers, is going to be relaxed. It will therefore be
necessary to join not only RIF for the immediate LAG, as is currently the
case, but also RIFs for VLAN netdevices upper to the LAG.

In this patch, extend mlxsw_sp_netdevice_router_join_lag() to walk the
uppers of a LAG being joined, and also join any VLAN ones.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>