PCI: Workaround IDT switch ACS Source Validation erratum

Some IDT switches incorrectly flag an ACS Source Validation error on
completions for config read requests even though PCIe r4.0, sec 6.12.1.1,
says that completions are never affected by ACS Source Validation.  Here's
the text of IDT 89H32H8G3-YC, erratum #36:

  Item #36 - Downstream port applies ACS Source Validation to Completions
  Section 6.12.1.1 of the PCI Express Base Specification 3.1 states that
  completions are never affected by ACS Source Validation.  However,
  completions received by a downstream port of the PCIe switch from a
  device that has not yet captured a PCIe bus number are incorrectly
  dropped by ACS Source Validation by the switch downstream port.

  Workaround: Issue a CfgWr1 to the downstream device before issuing the
  first CfgRd1 to the device.  This allows the downstream device to capture
  its bus number; ACS Source Validation no longer stops completions from
  being forwarded by the downstream port.  It has been observed that
  Microsoft Windows implements this workaround already; however, some
  versions of Linux and other operating systems may not.

When doing the first config read to probe for a device, if the device is
behind an IDT switch with this erratum:

  1. Disable ACS Source Validation if enabled
  2. Wait for device to become ready to accept config accesses (by using
     the Config Request Retry Status mechanism)
  3. Do a config write to the endpoint
  4. Enable ACS Source Validation (if it was enabled to begin with)

The workaround suggested by IDT is basically only step 3, but we don't know
when the device is ready to accept config requests.  That means we need to
do config reads until we receive a non-Config Request Retry Status, which
means we need to disable ACS SV temporarily.

Signed-off-by: James Puthukattukaran <james.puthukattukaran@oracle.com>
[bhelgaas: changelog, clean up whitespace, fold in unused variable fix
from Anders Roxell <anders.roxell@linaro.org>]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>