rtc: test: do not use assignment in if condition

Fix checkpatch error:
drivers/rtc/rtc-test.c:155: ERROR: do not use assignment in if condition
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-test.c

@@ -152,7 +152,8 @@ static int __init test_init(void)
 {
 	int i, err;
 
-	if ((err = platform_driver_register(&test_driver)))
+	err = platform_driver_register(&test_driver);
+	if (err)
 		return err;
 
 	err = -ENOMEM;