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>