selftests/net: toeplitz: fix udp option

Tiny fix. Option -u ("use udp") does not take an argument.

It can cause the next argument to silently be ignored.

Fixes: 5ebfb4cc ("selftests/net: toeplitz test")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>