pxa168_eth: fix race in transmit path.

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

Cc: stable@kernel.org
Cc: Sachin Sanap <ssanap@marvell.com>
Cc: Zhangfei Gao <zgao6@marvell.com>
Cc: Philip Rakity <prakity@marvell.com>
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>