net/pcap: remove Rx queue argument necessity

net/pcap: remove Rx queue argument necessity

Previously in the PCAP PMD queues has to be defined as RxQ and TxQ
pairs, even if the need is only Rx or only Tx:
 "--vdev net_pcap0,tx_pcap=tx.pcap,rx_pcap=rx.pcap"

Following commit enabled only providing Rx queue, and if Tx queue is
not provided PMD drops the Tx packets automatically:
Commit a3f5252e5cbd ("net/pcap: enable infinitely Rx a pcap file")
 "--vdev net_pcap0,rx_pcap=rx.pcap"

This commit enables same thing for Rx queue, user no more have to
provide a Rx queue (rx_iface or rx_pcap), for this case a dummy Rx
burst function is used which doesn't return any packet at all:
 "--vdev net_pcap0,tx_pcap=tx.pcap"

This makes only saving packets to a pcap file use case easy.

When both Rx and Tx queues are missing PMD will return an error.
(Single interface is still supported: "--vdev net_pcap0,iface=eth0")

Signed-off-by: Aideen McLoughlin <aideen.mcloughlin@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>