net: dsa: setup and teardown switches

This patches brings no functional changes. It removes the unused dst
argument from the dsa_ds_apply and dsa_ds_unapply functions, rename them
to dsa_switch_setup and dsa_switch_teardown for a more explicit scope.

This clarifies the steps of the setup or teardown of a switch fabric.

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