When input only Rx interface argument 'rx_iface_in' provided, current
code assigns 'eth_null_rx' burst function by mistake and no packet
received as a result.
Like in following usage no packets received from physical interface:
"--vdev net_pcap0,rx_iface_in=eth0,tx_iface=eth0"
Fixing the burst function assignment when 'rx_iface_in' argument is used