Save MSI controller in pci_sys_data instead of assigning MSI controller
pointer to every PCI bus in .add_bus().

[bhelgaas: use xilinx_pcie_msi_chip, not xilinx_pcie_msi_controller]
Signed-off-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>