net: fec: fix missing napi_disable call

Commit dc975382 introduces napi support
but never calls napi_disable. This will generate a kernel oops
(kernel BUG at include/linux/netdevice.h:473!) every time, when
ndo_stop is called followed by ndo_start.
Add the missing napi_diable call.
Signed-off-by: Georg Hofmann <georg@hofmannsweb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec.c

@@ -1441,6 +1441,7 @@ fec_enet_close(struct net_device *ndev)
 	struct fec_enet_private *fep = netdev_priv(ndev);
 
 	/* Don't know what to do yet. */
+	napi_disable(&fep->napi);
 	fep->opened = 0;
 	netif_stop_queue(ndev);
 	fec_stop(ndev);