dpif-netdev.at: Fix partial offloading test cases failure.

dpif-netdev.at: Fix partial offloading test cases failure.

Some partial offloading test cases are failing inconsistently. The root
cause is that dummy netdev is assigned with "linux_tc" offloading API.
dpif-netdev - partial hw offload - dummy
dpif-netdev - partial hw offload - dummy-pmd
dpif-netdev - partial hw offload with packet modifications - dummy
dpif-netdev - partial hw offload with packet modifications - dummy-pmd

This patch fixes this issue by changing 'options:ifindex=1' to some big
value. It is a workaround to make "linux_tc" init flow api failure. All
above cases can pass consistently after applying this patch.

Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>