sky2: Fix oops in sky2_xmit_frame() after TX timeout

sky2: Fix oops in sky2_xmit_frame() after TX timeout

[ Upstream commit 9db2f1bec36805e57a003f7bb90e003815d96de8 ]

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: Berck E. Nash <flyboy@gmail.com>
Tested-by: Berck E. Nash <flyboy@gmail.com>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>