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

Similar to commit be809424 ("selftests: bonding: do not set port down
before adding to bond"). The bond-arp-interval-causes-panic test failed
after commit a4abfa62 ("net: rtnetlink: Enslave device before bringing
it up") as the kernel will set the port down _after_ adding to bond if setting
port down specifically.

Fix it by removing the link down operation when adding to bond.

Fixes: 2ffd5732

 ("selftests: bonding: cause oops in bond_rr_gen_slave_id")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Tested-by: Benjamin Poirier <benjamin.poirier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>