dp83640: use proper function to free transmit time stamping packets

The previous commit enforces a new rule for handling the cloned packets
for transmit time stamping. These packets must not be freed using any other
function than skb_complete_tx_timestamp. This commit fixes the one and only
driver using this API.

The driver first appeared in v3.0.

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