net: fec: fix time stamping logic after napi conversion

Commit dc975382 "net: fec: add napi support to improve proformance"
converted the fec driver to the napi model. However, that commit
forgot to remove the call to skb_defer_rx_timestamp which is only
needed in non-napi drivers.

(The function napi_gro_receive eventually calls netif_receive_skb,
which in turn calls skb_defer_rx_timestamp.)

This patch should also be applied to the 3.9 and 3.10 kernels.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>