net: dsa: qca8k: correctly handle mdio read error

Restore original way to handle mdio read error by returning 0xffff.
This was wrongly changed when the internal_mdio_read was introduced,
now that both legacy and internal use the same function, make sure that
they behave the same way.

Fixes: ce062a0a ("net: dsa: qca8k: fix kernel panic with legacy mdio mapping")
Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>