selftests: mptcp: fix ACKRX debug message

Info from received MPCapable SYN were printed instead of the ones from
received MPCapable 3rd ACK.

Fixes: fed61c4b

 ("selftests: mptcp: make 2nd net namespace use tcp syn cookies unconditionally")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>