tty/serial: digicolor: Fix digicolor-usart already registered warning

When modprobe/rmmod/modprobe module, if platform_driver_register() fails,
the kernel complained,

proc_dir_entry 'driver/digicolor-usart' already registered
WARNING: CPU: 1 PID: 5636 at fs/proc/generic.c:360 proc_register+0x19d/0x270

Fix this by adding uart_unregister_driver() when platform_driver_register() fails.

Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Kefeng Wang <[e-mail]>
Acked-by: Baruch Siach <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/digicolor-usart.c

···
 	if (ret)
 		return ret;
 
-	return platform_driver_register(&digicolor_uart_platform);
+	ret = platform_driver_register(&digicolor_uart_platform);
+	if (ret)
+		uart_unregister_driver(&digicolor_uart);
+
+	return ret;
 }
 module_init(digicolor_uart_init);