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>