pktgen: Fix delay handling

After last pktgen changes, delay handling is wrong.

pktgen actually sends packets at full line speed.

Fix is to update pkt_dev->next_tx even if spin() returns early,
so that next spin() calls have a chance to see a positive delay.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>