app/testpmd: fix latency stats deinit on signal

app/testpmd: fix latency stats deinit on signal

On receiving signal, testpmd showing warning as
"LATENCY_STATS: failed to remove Rx/Tx callback"
because rte_latencystats_uninit is called without
checking if latencystats is enabled or not.

After this fix, rte_latencystats_uninit will be
called only if latencystats is enabled.

Fixes: 62d3216d6194 ("app/testpmd: add latency statistics calculation")
Cc: stable@dpdk.org
Signed-off-by: Amit Gupta <agupta3@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>