net: dsa: qca8k: check return value of read functions correctly

Current return type of qca8k_mii_read32() and qca8k_read() are
unsigned, it can't be negative, so the return value check is
unuseful. For check the return value correctly, change return
type of the read functions and add a output parameter to store
the read value.

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