net: mtk_eth_soc: delete an unneeded variable

There is already an "int err" declared at the start of the function so
re-use that instead of declaring a shadow err variable.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211217070735.GC26548@kili


Signed-off-by: Jakub Kicinski <kuba@kernel.org>