selftests: tc-testing: fix tunnel_key failure if dst_port is unspecified

After commit 1c25324c

 ("net/sched: act_tunnel_key: Don't dump dst port
if it wasn't set"), act_tunnel_key doesn't dump anymore the destination
port, unless it was explicitly configured. This caused systematic failures
in the following TDC test case:

 7a88 - Add tunnel_key action with cookie parameter

Avoid matching zero values of TCA_TUNNEL_KEY_ENC_DST_PORT to let the test
pass again.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>