net: dsa: qca8k: fix kernel panic with legacy mdio mapping

When the mdio legacy mapping is used the mii_bus priv registered by DSA
refer to the dsa switch struct instead of the qca8k_priv struct and
causes a kernel panic. Create dedicated function when the internal
dedicated mdio driver is used to properly handle the 2 different
implementation.

Fixes: 759bafb8 ("net: dsa: qca8k: add support for internal phy and internal mdio")
Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>