r8152: fix accessing skb after napi_gro_receive

Fix accessing skb after napi_gro_receive which is caused by
commit 47922fcd ("r8152: support skb_add_rx_frag").

Fixes: 47922fcd

 ("r8152: support skb_add_rx_frag")
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>