selftests/net: fix warnings in TFO key rotation selftest

One warning each on signedness, unused variable and return type.

Fixes: 10fbcdd1 ("selftests/net: add TFO key rotation selftest")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>