devlink: hold the instance lock in port_split / port_unsplit callbacks

Let the core take the devlink instance lock around port splitting
and remove the now redundant locking in the drivers.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>