net: ethernet: mtk_eth_soc: fix error checking in mtk_mac_config()

stable inclusion
from stable-v5.10.94
commit 819e76bc572e373db88956ddc219e9976b16a10f
bugzilla: https://gitee.com/openeuler/kernel/issues/I531X9

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=819e76bc572e373db88956ddc219e9976b16a10f



--------------------------------

commit 214b3369 upstream.

Clang static analysis reports this problem
mtk_eth_soc.c:394:7: warning: Branch condition evaluates
  to a garbage value
                if (err)
                    ^~~

err is not initialized and only conditionally set.
So intitialize err.

Fixes: 7e538372 ("net: ethernet: mediatek: Re-add support SGMII")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>