PCI/AER: Consolidate HEST error source parsers

aer_hest_parse() and aer_hest_parse_aff() are almost identical.  We use
aer_hest_parse() to check the ACPI_HEST_FIRMWARE_FIRST flag for a specific
device, and we use aer_hest_parse_aff() to check to see if any device sets
the flag.

This drops aer_hest_parse_aff() and enhances aer_hest_parse() so it
collects the union of the PCIe ACPI_HEST_FIRMWARE_FIRST flag settings when
no specific device is supplied.

No functional change.

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