fec_mpc52xx: fix timestamp filtering

commit 9ca3cc6f3026946ba655e863ca2096339e667639 upstream.

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>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>