e1000e: add Tx timeout factor for 100Mbps

On some devices (e.g. 82578) not having a Tx timeout factor when linked at
100Mbps can cause false reports of hardware hangs on busy hubs.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>