Fixed 'tc qdisc show' for tbf when latency<0

Fixed 'tc qdisc show' for tbf when latency<0

When limit<burst latency becomes <0, for example:
 # tc qdisc add dev eth0 root handle 1: tbf limit 100K burst 256K rate 256kbit
 # tc qdisc show
 qdisc tbf 1: dev eth0 root refcnt 2 rate 256Kbit burst 256Kb lat 4290.0s

If latency<0 there is no reason to show it. Limit will be printed instead of
latency when latency<0:
 # tc qdisc show
 qdisc tbf 1: dev eth0 root refcnt 2 rate 256Kbit burst 256Kb limit 100Kb

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>