tile/PCI: use cached pci_dev->pcie_mpss to simplify code

The PCI core caches the "PCIe Max Payload Size Supported" in
pci_dev->pcie_mpss, so use that instead of pcie_capability_read_dword().

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>