net/virtio: fix device configure without jumbo Rx offload

net/virtio: fix device configure without jumbo Rx offload

Use max-pkt-len only if jumbo frames offload is requested
since otherwise this field isn't valid.

Fixes: 8b90e4358112 ("net/virtio: set offload flag for jumbo frames")
Fixes: 4e8169eb0d2d ("net/virtio: fix Rx scatter offload")
Cc: stable@dpdk.org
Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>