dcb: Add stub routines for !CONFIG_DCB

To avoid ifdefs in the other code that supports DCB notifiers
add stub routines. This method seems popular in other net code
for example 8021Q.

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