e1000e: Set HW FIFO minimum pointer gap for non-gig speeds

Based on feedback from HW team, the configured value of the internal PHY
HW FIFO pointer gap was incorrect for non-gig speeds.
This patch provides the correct configuration.

Signed-off-by: Raanan Avargil <raanan.avargil@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>