ACPI / APEI: Fix parsing HEST that includes Deferred Machine Check subtable

ACPI 6.2 includes a new definition for a Deferred Machine Check "DMC"
subtable.

The definition of this subtable was included in following commit:

c042933d (ACPICA: Add support for new HEST subtable)

However, the HEST parsing function was not updated to include this new
subtable. Therefore, Linux will fail to parse the HEST on systems that
include a DMC entry.

Add the length check for the new DMC subtable so that HEST parsing
doesn't fail on systems that include it.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>