mediatek: unlock on error in mtk_tx_map()

There was a missing unlock on the error path.

Fixes: 656e705243 ('net-next: mediatek: add support for MT7623 ethernet')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -661,6 +661,8 @@ err_dma:
 		itxd = mtk_qdma_phys_to_virt(ring, itxd->txd2);
 	} while (itxd != txd);
 
+	spin_unlock_irqrestore(&eth->page_lock, flags);
+
 	return -ENOMEM;
 }