eth: mlxsw: switch to explicit locking for port registration

Explicitly lock the devlink instance and use devl_ API.

This will be used by the subsequent patch to invoke
.port_split / .port_unsplit callbacks with devlink
instance lock held.

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