ixgbe: DCB, X540 devices do not respond to pause frames

DCB enabled X540 devices are not responding to pause frames
due to a missing register set that was added for these
devices that did not exist in other devices.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>