net: dsa: setup a tree when adding a switch to it

Now that the tree setup is centralized, we can simplify the code a bit
more by setting up or tearing down the tree directly when adding or
removing a switch to/from it.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>