ACPI: APEI: Add is_generic_error() to identify GHES sources

Refactor duplicated GHES identity logic into is_generic_error().

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Reviewed-by: Robert Richter <rrichter@amd.com>
Co-developed-by: Terry Bowman <terry.bowman@amd.com>
Signed-off-by: Terry Bowman <terry.bowman@amd.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>