MIPS: Loongson: PCI: use existing mips_io_port_base

mips_io_port_base is initialized via set_io_port_base() in
arch/mips/lemote/lm2e/setup.c, we can use it directly here.

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