net: devlink: make devlink_dpipe_headers_register() return void

The return value is not used, so change the return value type to void.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>