e1000e: do not write SmartSpeed register bits on parts without support

The driver was accessing register bits for features on parts that do
not support that feature.  This could cause problems in the hardware.

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>