x86, efi: Cleanup config table walking

Trivial cleanup, move guid and table pointers to local copies to
make the code cleaner.

Signed-off-by: Olof Johansson <olof@lixom.net>
Link: http://lkml.kernel.org/r/1329081869-20779-4-git-send-email-olof@lixom.net
Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>