mips: Vr41xx: add missing iounmap() on error in vr41xx_pciu_init()

add missing iounmap() of pciu_base on error when failed to init
io_map_base.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>