sky2: Lock transmit queue while disabling device

netif_device_detach() does not take the tx_lock, so it's possible that
a call to sky2_xmit_frame is still in progress after
netif_device_detach() is complete.

Take netif_tx_lock() to make sure all transmits have stopped while
we're disabling the devices and that no other CPU is still
transmitting a frame after we've disabling the device.

Proposed fix for "sky2 panic under load" reported by Berck E. Nash.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>