e1000e: Enable DMA Burst Mode on 82574 by default.

Performance testing has shown that enabling DMA burst on 82574
improves performance on small packets, so enable it by default.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>