MIPS: PCI: Use PCI Express Capability accessors

Use PCI Express Capability access functions to simplify MIPS PCIe code.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: David Daney <david.daney@cavium.com>