ixgbe: Set Priority Flow Control low water threshhold for DCB

This sets the low water threshhold for priority flow control for 82598
and 82599 controllers in DCB mode.

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>