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>