ixgbe: Fix the DCB PFC thresholds for 82599

The thresholds for the DCB priority flow control are incorrect for 82599.
This fixes the thresholds to be correct.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>