mtd: elm: compile suspend/resume only with PM_SLEEP

Fixes:
drivers/mtd/devices/elm.c:480:12: warning: 'elm_suspend' defined but not used [-Wunused-function]
drivers/mtd/devices/elm.c:488:12: warning: 'elm_resume' defined but not used [-Wunused-function]
Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/devices/elm.c

@@ -418,6 +418,7 @@ static int elm_remove(struct platform_device *pdev)
 	return 0;
 }
 
+#ifdef CONFIG_PM_SLEEP
 /**
  * elm_context_save
  * saves ELM configurations to preserve them across Hardware powered-down
@@ -509,6 +510,7 @@ static int elm_resume(struct device *dev)
 	elm_context_restore(info);
 	return 0;
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(elm_pm_ops, elm_suspend, elm_resume);