mlxsw: spectrum: Add debug prints

For debug purposes, it's useful to know the order in which the driver
responds to changes in the topology of its upper devices.

Add debug prints to signal these events.

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>