o update acpi_gbl_FADT.header.length after
the original value is used to check for old revisions.

https://bugzilla.kernel.org/show_bug.cgi?id=42700
https://bugzilla.redhat.com/show_bug.cgi?id=727865

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
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