PCI: dwc: ep: Flush cached MSI write before unmapping the iATU

In the Linux kernel, the following vulnerability has been resolved:

PCI: dwc: ep: Flush cached MSI write before unmapping the iATU

The MSI-X path already flushes any posted MSI-X write before tearing down
its iATU mapping. That was added by commit c22533c66cca ("PCI: dwc: ep:
Flush MSI-X write before unmapping its ATU entry") to make sure the write
reaches the Root Complex before the outbound window that translates it
disappears.

The MSI path has the same problem but no equivalent flush. When the
Endpoint driver caches an MSI target address and later observes that the
Root Complex has changed it, dw_pcie_ep_raise_msi_irq() unmaps the existing
iATU entry and reprograms it for the new address. Between the last MSI
writel() and the unmap there may still be a posted write sitting in the
fabric, and unmapping the iATU entry can drop or misroute that write.

Fix this by reading back from the mapped MSI window before the unmap. The
readback drains any posted MSI writes through the same iATU entry that
mapped them, which is the same logic the MSI-X path uses.

[mani: commit log]