net: microchip: sparx5: Improve the error handling for linked rules

Ensure that an error is returned if the VCAP instance was not found.
The chain offset (diff) is allowed to be zero as this just means that the
user did not request rules to be linked.

Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>