PCI/AER: Ignore non-PCIe AER error sources in aer_hest_parse()

aer_set_firmware_first() searches the HEST for an error source descriptor
matching the specified PCI device.  It uses the apei_hest_parse() iterator
to call aer_hest_parse() for every descriptor in the HEST.

Previously, aer_hest_parse() incorrectly assumed every descriptor was for a
PCIe error source.  This patch adds a check to avoid that error.

[bhelgaas: factor check into helper, use in aer_hest_parse_aff(), changelog]
Signed-off-by: Betty Dall <betty.dall@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>