e1000e: fix flow-control thresholds for jumbo frames on 82579/I217/I218

The previous static flow-control thresholds were causing unnecessary pause
packets to be transmitted when jumbo frames are configured reducing the
throughput.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>