[IPROUTE]: tbf: fix latency printing

The calculated latency is already in usecs, the additional tick2usec
conversion breaks the calculation with jiffies or tsc clock source.

Example:

# tc qdisc add dev dummy0 root tbf latency 20ms burst 10k rate 50mbit
# tc qdisc show dev dummy0
qdisc tbf 8002: rate 50000Kbit burst 10Kb lat 15.4ms

Fixed:

# tc qdisc show dev dummy0
qdisc tbf 8002: rate 50000Kbit burst 10Kb lat 20ms
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>