net: ethernet: mtk_eth_soc: fix possible NULL pointer dereference in mtk_xdp_run

Fix possible NULL pointer dereference in mtk_xdp_run() if the
ebpf program returns XDP_TX and xdp_convert_buff_to_frame routine fails
returning NULL.

Fixes: 5886d26f

 ("net: ethernet: mtk_eth_soc: add xmit XDP support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/627a07d759020356b64473e09f0855960e02db28.1660659112.git.lorenzo@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>