sky2: Avoid rewinding sky2->tx_prod

Keep sky2->tx_prod consistent since int might be examined by
an softirq poll or restart.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>