net: dsa: qca8k: add missing check return value in qca8k_phylink_mac_config()

Now we can check qca8k_read() return value correctly, so if
it fails, we need return directly.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>