net: dsa: setup and teardown tree

This commit provides better scope for the DSA tree setup and teardown
functions. It renames the "applied" bool to "setup" and print a message
when the tree is setup, as it is done during teardown.

At the same time, check dst->setup in dsa_tree_setup, where it is set to
true.

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