serial: altera_uart: call iounmap() at driver remove

The driver calls ioremap() in the probe function but doesn't call
iounmap() in the remove function correspondingly. Do so now.

Follow commit 5c9d6abe ("serial: altera_jtaguart: adding iounmap()")

Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>