selftests: bonding: do not set port down before adding to bond

Before adding a port to bond, it need to be set down first. In the
lacpdu test the author set the port down specifically. But commit
a4abfa62 ("net: rtnetlink: Enslave device before bringing it up")
changed the operation order, the kernel will set the port down _after_
adding to bond. So all the ports will be down at last and the test failed.

In fact, the veth interfaces are already inactive when added. This
means there's no need to set them down again before adding to the bond.
Let's just remove the link down operation.

Fixes: a4abfa62

 ("net: rtnetlink: Enslave device before bringing it up")
Reported-by: Zhengchao Shao <shaozhengchao@huawei.com>
Closes: https://lore.kernel.org/netdev/a0ef07c7-91b0-94bd-240d-944a330fcabd@huawei.com/
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20230817082459.1685972-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>