mlxsw: spectrum: Sync PVID vPort LAG status

When VLAN devices are created on top of LAG, their underlying vPorts are
configured correctly with LAG membership.

However, the PVID vPort is implicit and already present when the port
netdev is put under LAG, so its LAG membership is never set. Set it
correctly when joining / leaving LAG.

This didn't matter until now, but we are going to introduce support for
router interfaces (RIFs), which need to take into account LAG membership.

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>