serial: efm32: add module_exit

Add missing specification of efm32_uart_exit as module_exit.

This fixes the following compilation warning:

  drivers/tty/serial/efm32-uart.c:840:123: warning: ‘efm32_uart_exit’ defined but not used [-Wunused-function]
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Jiri Slaby <jslaby@suse.cz>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/efm32-uart.c

@@ -842,6 +842,7 @@ static void __exit efm32_uart_exit(void)
 	platform_driver_unregister(&efm32_uart_driver);
 	uart_unregister_driver(&efm32_uart_reg);
 }
+module_exit(efm32_uart_exit);
 
 MODULE_AUTHOR("Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>");
 MODULE_DESCRIPTION("EFM32 UART/USART driver");