pxa168_eth: convert skb->end into skb_end_pointer(skb)

The change set of 4305b541

, "[SK_BUFF]: Convert skb->end to sk_buff_data_t"
converted skb->end from pointer type to sk_buff_data_t.
The pointed value should be accessed via skb_end_pointer().

Since arm 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 tested only.

Cc: Simon Horman <horms@verge.net.au>
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>