Make the do_index and do_op functions static.

They are used only internally by the 842 decompression function,
and should be static.
Reported-By: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NDan Streetman <ddstreet@ieee.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>