devlink: convert reporters dump to devlink_nl_instance_iter_dump()

Benefit from recently introduced instance iteration and convert
reporters .dumpit generic netlink callback to use it.

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