serial: MIPS: lantiq: use devm_iounmap instead of iounmap

port->membase was allocated using devm_ioremap_nocache, so ideally
we should unmap it using devm_iounmap. but it was using iounmap.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>