sky2: Avoid race in sky2_change_mtu

netif_stop_queue does not ensure all in-progress transmits are complete,
 so use netif_tx_disable() instead.

Secondly, make sure NAPI polls are disabled before stopping the tx queue,
 otherwise sky2_status_intr might trigger a TX queue wakeup between when
 we stop the queue and NAPI is disabled.

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