netdev-dpdk: Fix sw stats perf drop.

netdev-dpdk: Fix sw stats perf drop.

Accessing the sw stats in the vhost datapath of a PVP test
can incur a performance drop of ~2%.

Most of the time these stats will just be getting zero added
to them. By checking if there is a non-zero update first, we
can avoid accessing them when they won't be updated and avoid
the performance drop.

Fixes: 2f862c712e52 ("netdev-dpdk: Detailed packet drop statistics.")
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>