pktgen: fix errata in show results

The units in show_results in pktgen were not correct.
The results are in usec but it was displayed nsec.

Reported-by: Jong-won Lee <ljw@handong.edu>
Signed-off-by: Daniel Turull <daniel.turull@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>