e1000e: increase minimum frame size allowed

Setting an MTU value below 68 was disabling the network connection and
would not reconnect until the driver was reloaded.  Prevent changing the
MTU to anything below 68.

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>