net: dsa: use switchdev handle helpers

Get rid of the dsa_slave_switchdev_port_{attr_set,obj}_event functions
in favor of the switchdev_handle_port_{attr_set,obj_add,obj_del}
helpers which recurse into the lower devices of the target interface.

This has the benefit of being aware of the operations made on the
bridge device itself, where orig_dev is the bridge, and dev is the
slave. This can be used later to configure the hardware switches.

Only VLAN and (port) MDB objects not directly targeting the slave
device are unsupported at the moment, so skip this case in their
respective case statements.

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