devlink: remove start variables from dumps

The start variables made the code clearer when we had to access
cb->args[0] directly, as the name args doesn't explain much.
Now that we use a structure to hold state this seems no longer
needed.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>