fec_mpc52xx: fix timestamp filtering

skb_defer_rx_timestamp was called with a freshly allocated skb but must
be called with rskb instead.

Signed-off-by: Stephan Gatzka <stephan@gatzka.org>
Cc: stable <stable@vger.kernel.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>