PCI/AER: Don't parse HEST table for non-PCIe devices

AER is a PCIe-only capability, so there's no point in trying to match
a HEST PCIe structure with a non-PCIe device.

Previously, a HEST global AER bridge entry (type 8) could incorrectly
match *any* bridge, even a legacy PCI-PCI bridge, and a non-global
HEST entry could match a legacy PCI device.

Tested-by: Betty Dall <betty.dall@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>