lib: make lib/842 decompress functions static

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: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>