Revert "net: dsa: qca8k: cache lo and hi for mdio write"

This reverts commit 2481d206.

The Documentation is very confusing about the topic.
The cache logic for hi and lo is wrong and actually miss some regs to be
actually written.

What the Documentation actually intended was that it's possible to skip
writing hi OR lo if half of the reg is not needed to be written or read.

Revert the change in favor of a better and correct implementation.

Reported-by: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Cc: stable@vger.kernel.org # v5.18+
Signed-off-by: David S. Miller <davem@davemloft.net>