devlink: remove linecard reference counting

As long as the linecard life time is protected by devlink instance
lock, the reference counting is no longer needed. Remove it.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>