dcbnl: make get_app handling symmetric for IEEE and CEE DCBx

The IEEE get/set app handlers use generic routines and do not
require the net_device to implement the dcbnl_ops routines. This
patch makes it symmetric so user space and drivers do not have
to handle the CEE version and IEEE DCBx versions differently.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>