net/fec: remove the use of "index" which is legacy

The "index" becomes legacy since fep->pdev->id starts working
to identify the instance.

Moreover, the call of fec_enet_init(ndev, 0) always passes 0
to fep->index. This makes the following code in fec_get_mac buggy.

	/* Adjust MAC if using default MAC address */
	if (iap == fec_mac_default)
		dev->dev_addr[ETH_ALEN-1] = fec_mac_default[ETH_ALEN-1] + fep->index;

It may be the time to remove "index" and use fep->pdev->id instead.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>