net: ethernet: mtk-star-emac: return ok when xmit drops

The ndo_start_xmit() method must return NETDEV_TX_OK if the DMA mapping
fails, after freeing the socket buffer.
Fix the mtk_star_netdev_start_xmit() function accordingly.

Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20201112084833.21842-1-vincent.stehle@laposte.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mediatek/mtk_star_emac.c

@@ -1053,7 +1053,7 @@ static int mtk_star_netdev_start_xmit(struct sk_buff *skb,
 err_drop_packet:
 	dev_kfree_skb(skb);
 	ndev->stats.tx_dropped++;
-	return NETDEV_TX_BUSY;
+	return NETDEV_TX_OK;
 }
 
 /* Returns the number of bytes sent or a negative number on the first