e1000e: fix the build error when PM is disabled

The commit 28002099 (e1000e: Refactor PM flows) changed the
SET_SYSTEM_SLEEP_PM_OPS to open-coded assignment, but forgot to
protect them with CONFIG_PM_SLEEP. Then cause the following build
error when PM is disabled:
drivers/net/ethernet/intel/e1000e/netdev.c:7079:13:
error: 'e1000e_pm_suspend' undeclared here (not in a function)
  .suspend = e1000e_pm_suspend,
             ^
drivers/net/ethernet/intel/e1000e/netdev.c:7080:13:
error: 'e1000e_pm_resume' undeclared here (not in a function)
  .resume  = e1000e_pm_resume,
             ^
drivers/net/ethernet/intel/e1000e/netdev.c:7082:11:
error: 'e1000e_pm_thaw' undeclared here (not in a function)
  .thaw  = e1000e_pm_thaw,
           ^
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>