e1000e: Fix CRC errors with jumbo traffic

Modifying the jumbo frame workaround for 82579, i217 and i218 client parts
to increase the gap between the read and write pointers in the Tx FIFO.

Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>