[SERIAL] imx: Fix missed platform_driver_unregister

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/serial/imx.c

@@ -994,7 +994,7 @@ static int __init imx_serial_init(void)
 static void __exit imx_serial_exit(void)
 {
 	uart_unregister_driver(&imx_reg);
-	driver_unregister(&serial_imx_driver);
+	platform_driver_unregister(&serial_imx_driver);
 }
 
 module_init(imx_serial_init);