net/pcap: fix input only Rx

net/pcap: fix input only Rx

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

Fixes: f14a94591504 ("net/pcap: remove Rx queue argument necessity")
Cc: stable@dpdk.org
Reported-by: Muthurajan Jayakumar <muthurajan.jayakumar@intel.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>