dcb: data center bridging ops should be r/o

The data center bridging ops structure can be const

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>