[PKTGEN]: Fix multiqueue oops.

Initially pkt_dev can be NULL this causes netif_subqueue_stopped to
oops. The patch below should cure it. But maybe the pktgen TX logic
should be reworked to better support the new multiqueue support.

Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>