app/testpmd: fix CPU cycles per packet stats on Tx modes

app/testpmd: fix CPU cycles per packet stats on Tx modes

In txonly and flowgen forwarding mode, calculating CPU per packets with
total received packets is not accurate. Use total transmitted packets
for these cases.

The error output under txonly mode:
testpmd> show fwd stats all

---------------------- Forward statistics for port 0  -------------------
RX-packets: 0              RX-dropped: 0             RX-total: 0
TX-packets: 3582891927     TX-dropped: 401965824     TX-total: 3984857751
TX-bursts : 86381636 [0% of 0 pkts + 85% of 64 pkts + 15% of 32 pkts]
-------------------------------------------------------------------------

---------------------- Forward statistics for port 1  -------------------
RX-packets: 1              RX-dropped: 394351696     RX-total: 394351697
TX-packets: 3582890632     TX-dropped: 401965568     TX-total: 3984856200
TX-bursts : 86381679 [0% of 0 pkts + 85% of 64 pkts + 15% of 32 pkts]
-------------------------------------------------------------------------

+++++++++++++++ Accumulated forward statistics for all ports+++++++++++++
RX-packets: 1              RX-dropped: 394351696     RX-total: 394351697
TX-packets: 7165782559     TX-dropped: 803931392     TX-total: 7969713951
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

CPU cycles/packet=54984156291.00 \
(total cycles=54984156291 / total RX packets=1) at 200 MHz Clock

Fixes: 53324971a14e ("app/testpmd: display/clear forwarding stats on demand")
Cc: stable@dpdk.org
Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>