devlink: add instance lock assertion in devl_is_registered()

After region and linecard lock removals, this helper is always supposed
to be called with instance lock held. So put the assertion here and
remove the comment which is no longer accurate.

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>