e1000e: Enclose e1000e_pm_thaw() with CONFIG_PM_SLEEP

Fix following compilation warning:
drivers/net/ethernet/intel/e1000e/netdev.c:6238:12: warning
‘e1000e_pm_thaw’ defined but not used [-Wunused-function]
 static int e1000e_pm_thaw(struct device *dev)
            ^
Signed-off-by: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>