serial: altera_jtaguart: adding iounmap()

The driver does ioremap(port->mapbase, ALTERA_JTAGUART_SIZE),
but there is no any iounmap().

Found by Linux Driver Verification project (linuxtesting.org).

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