sky2: Fix oops in sky2_xmit_frame() after TX timeout

During TX timeout procedure dev could be awoken too early, e.g. by
sky2_complete_tx() called from sky2_down(). Then sky2_xmit_frame()
can run while buffers are freed causing an oops. This patch fixes it
by adding netif_device_present() test in sky2_tx_complete().

Fixes: http://bugzilla.kernel.org/show_bug.cgi?id=14925

With debugging by: Mike McCormack <mikem@ring3k.org>
Reported-by: NBerck E. Nash <flyboy@gmail.com>
Tested-by: NBerck E. Nash <flyboy@gmail.com>
Signed-off-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sky2.c

@@ -1844,7 +1844,8 @@ static void sky2_tx_complete(struct sky2_port *sky2, u16 done)
 	sky2->tx_cons = idx;
 	smp_mb();
 
-	if (tx_avail(sky2) > MAX_SKB_TX_LE + 4)
+	/* Wake unless it's detached, and called e.g. from sky2_down() */
+	if (tx_avail(sky2) > MAX_SKB_TX_LE + 4 && netif_device_present(dev))
 		netif_wake_queue(dev);
 }