PCI: pciehp: Disable in-band presence detect when possible

PCI: pciehp: Disable in-band presence detect when possible

The presence detect state (PDS) is normally a logical OR of in-band and
out-of-band (OOB) presence detect.  As of PCIe 4.0, there is the option to
disable in-band presence so that the PDS bit always reflects the state of
the out-of-band presence.

The recommendation of the PCIe spec is to disable in-band presence whenever
supported (PCIe r5.0, appendix I implementation note):

  Due to architectural issues, the in-band (Physical-Layer-based) portion
  of the PD mechanism is deprecated for use with async hot-plug. One issue
  is that in-band PD as architected does not detect adapter removal during
  certain LTSSM states, notably the L1 and Disabled States.  Another issue
  is that when both in-band and OOB PD are being used together, the
  Presence Detect State bit and its associated interrupt mechanism always
  reflect the logical OR of the inband and OOB PD states, and with some
  hot-plug hardware configurations, it is important for software to detect
  and respond to in-band and OOB PD events independently.  If OOB PD is
  being used and the associated DSP supports In-Band PD Disable, it is
  recommended that the In-Band PD Disable bit be Set, and the Presence
  Detect State bit and its associated interrupt mechanism be used
  exclusively for OOB PD.  As a substitute for in-band PD with async
  hot-plug, the reference model uses either the DPC or the DLL Link Active
  mechanism.

Link: https://lore.kernel.org/r/20191025190047.38130-2-stuart.w.hayes@gmail.com
[bhelgaas: move PCI_EXP_SLTCAP2 read earlier & print PCI_EXP_SLTCAP2_IBPD
value (suggested by Lukas)]
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>