kernel: mtk_eth_soc: fix memory leak in downstream patch

kernel: mtk_eth_soc: fix memory leak in downstream patch

Bc-bocun Chen of MediaTek has discovered a memory leak in the error path
in our downstream patch for mtk_eth_soc which adds support for the 10G
PCS and PHY paths of the MT7988 SoC.
Fix this by freeing the at this point already allocated netdev resources
before returning the error.

Fixes: 4cb6bd9a6d ("mediatek: switch to pending XFI 10G Ethernet drivers")
Reported-by: Bc-bocun Chen <bc-bocun.chen@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>