examples/l3fwd: fix packets lost when stopping

Not all tx ports was included in tx_port_id array, used to periodically
drain only available ports. This caused that some packets remain in buffer
when application stops to receiving packets.

Fixes: 52c97adc1f0f ("examples/l3fwd: fix exact match performance")

Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>