net: ethernet: mtk_eth_soc: unmap RX data before calling build_skb

Since build_skb accesses the data area (for initializing shinfo), dma unmap
needs to happen before that call

Signed-off-by: Felix Fietkau <nbd@nbd.name>
[Ilya: split build_skb cleanup fix into a separate commit]
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>