net: ethernet: mtk_eth_soc: remove duplicate if statements

It seems that there was something wrong with backport,
causing `if (err)` to appear twice in the same place.

Fixes: da86a634 ("net: ethernet: mtk_eth_soc: fix error handling in mtk_open()")
Cc: Liu Jian <liujian56@huawei.com>
Cc: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>