PCI/AER: Clear error status registers during enumeration and restore

AER errors might be recorded when powering-on devices.  These errors can be
ignored, so firmware usually clears them before the OS enumerates devices.
However, firmware is not involved when devices are added via hotplug, so
the OS may discover power-up errors that should be ignored.  The same may
happen when powering up devices when resuming after suspend.

Clear the AER error status registers during enumeration and resume.

[bhelgaas: changelog, remove repetitive comments]
Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>