x86, efi: Set runtime_version to the EFI spec revision

efi.runtime_version is erroneously being set to the value of the
vendor's firmware revision instead of that of the implemented EFI
specification. We can't deduce which EFI functions are available based
on the revision of the vendor's firmware since the version scheme is
likely to be unique to each vendor.

What we really need to know is the revision of the implemented EFI
specification, which is available in the EFI System Table header.

Cc: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: stable@vger.kernel.org # 3.7.x
Signed-off-by: Matt Fleming <matt.fleming@intel.com>