devlink: use devl_is_registered() helper instead xa_get_mark()

Instead of checking the xarray mark directly using xa_get_mark() helper
use devl_is_registered() helper which wraps it up. Note that there are
couple more users of xa_get_mark() left which are going to be handled
by the next patch.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>