mv643xx_eth: fix race in trasmit path.

Because the socket buffer is freed in the completion interrupt, it is not
safe to access it after submitting it to the hardware.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>