shell: Fix command completion logic

The original code (introduced by commit d5db3520) looked like this
before the last rewrite/cleanup:

       if (default_module != -1) {
               return (str == NULL) ? dest : -1;
       }

However with the cleanup the logic seems to have gotten reversed.

Fixes #8501

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>