firmware loader: fix compile failure if !PM

'return 0' should be added to fw_pm_notify if !PM because
return value of the funcion is defined as 'int'.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/firmware_class.c

@@ -1242,7 +1242,9 @@ static int fw_pm_notify(struct notifier_block *notify_block,
 #else
 static int fw_pm_notify(struct notifier_block *notify_block,
 			unsigned long mode, void *unused)
-{}
+{
+	return 0;
+}
 #endif
 
 static void __init fw_cache_init(void)