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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/digicolor-usart.c

@@ -541,7 +541,11 @@ static int __init digicolor_uart_init(void)
 	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);