e1000e: do not attempt to disable smart powerdown if not supported

During reset, the driver was attempting to disable the Smart Powerdown
feature even if the part does not support Smart Powerdown.  Check for
support before attempting to disable the feature.

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>