mailbox/omap: unregister mbox class

platform_driver_register() can fail here and we must unregister mbox
class.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/omap-mailbox.c

@@ -906,7 +906,11 @@ static int __init omap_mbox_init(void)
 	mbox_kfifo_size = max_t(unsigned int, mbox_kfifo_size,
 							sizeof(mbox_msg_t));
 
-	return platform_driver_register(&omap_mbox_driver);
+	err = platform_driver_register(&omap_mbox_driver);
+	if (err)
+		class_unregister(&omap_mbox_class);
+
+	return err;
 }
 subsys_initcall(omap_mbox_init);