selftests/txtimestamp: Fix an equals vs assign bug

This should be == instead of =.

Fixes: b52354aa

 ("selftests: expand txtimestamp with ipv6 dgram + raw and pf_packet")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>