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

[bhelgaas: use dw_pcie_msi_chip, not dw_pcie_msi_controller]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>