MIPS: Loongson: PCI: Clean up pcimap setup

Fixup the wrong original comment of pcimap, and make the source code more
understandable. and also, some new extra consideration is added in.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>