mv643xx_eth.c: convert skb->end into skb_end_poitner(skb)

The change set of 4305b541 "[SK_BUFF]: Convert skb->end to sk_buff_data_t"
converted skb->end from pointer to sk_buff_data_t.
The pointed value should be accessed via skb_end_pointer().

Since arm or ppc arch doesn't define NET_SKBUFF_DATA_USES_OFFSET,
skb->end is effectively pointer. So it doesn't cause a real problem.
But this patch is good for consistency.

Found by inspection. Compile test only.

Cc: Simon Horman <horms@verge.net.au>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mv643xx_eth.c

@@ -621,7 +621,7 @@ static int rxq_refill(struct rx_queue *rxq, int budget)
 
 		rx_desc = rxq->rx_desc_area + rx;
 
-		size = skb->end - skb->data;
+		size = skb_end_pointer(skb) - skb->data;
 		rx_desc->buf_ptr = dma_map_single(mp->dev->dev.parent,
 						  skb->data, size,
 						  DMA_FROM_DEVICE);