drivers/block/mg_disk.c: add CONFIG_PM_SLEEP to suspend/resume functions

Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following build
warning when CONFIG_PM_SLEEP is not selected. This is because sleep PM
callbacks defined by SIMPLE_DEV_PM_OPS are only used when the
CONFIG_PM_SLEEP is enabled.

drivers/block/mg_disk.c:783:12: warning: 'mg_suspend' defined but not used [-Wunused-function]
drivers/block/mg_disk.c:807:12: warning: 'mg_resume' defined but not used [-Wunused-function]

Signed-off-by: Jingoo Han <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/mg_disk.c

···
 	.getgeo = mg_getgeo
 };
 
+#ifdef CONFIG_PM_SLEEP
 static int mg_suspend(struct device *dev)
 {
 	struct mg_drv_data *prv_data = dev->platform_data;
···
 
 	return 0;
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(mg_pm, mg_suspend, mg_resume);