e1000e: i219 - Increase minimum FIFO read/write min gap

Due to clocking changes in the Skylake platform, there was i219
data corruption. To work around this, HW team reported the need
to increase the minimum gap between the PHY FIFO read and write pointers.

Signed-off-by: Yanir Lubetkin <yanirx.lubetkin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>