e1000e: Increase Tx timeout factor for 10Mbps

Prevent Tx hangs from happening on 10Mb flood ping by increasing the
timeout factor.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>