net: dsa: qca8k: remove leftover phy accessors

This belated patch implements Andrew Lunn's request of
"remove the phy_read() and phy_write() functions."
<https://lore.kernel.org/patchwork/comment/902734/>

While seemingly harmless, this causes the switch's user
port PHYs to get registered twice. This is because the
DSA subsystem will create a slave mdio-bus not knowing
that the qca8k_phy_(read|write) accessors operate on
the external mdio-bus. So the same "bus" gets effectively
duplicated.

Cc: stable@vger.kernel.org
Fixes: 6b93fb46

 ("net-next: dsa: add new driver for qca8xxx family")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>