net: ethernet: mtk-star-emac: fix wrong unmap in RX handling

mtk_star_dma_unmap_rx() should unmap the dma_addr of old skb rather than
that of new skb.
Assign new_dma_addr to desc_data.dma_addr after all handling of old skb
ends to avoid unexpected receive side error.

Fixes: f96e9641

 ("net: ethernet: mtk-star-emac: fix error path in RX handling")
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>