net: dsa: mv88e6xxx: Pass mii_bus to all PHY operations

In preparation for supporting multiple MDIO busses, pass the mii_bus
structure to all PHY operations. It will in future then be clear on
which MDIO bus the operation should be performed.

For reads/write from phylib, the mii_bus is readily available. However
some internal code also access the PHY, e.g. for EEE and SERDES. Make
this code use the one and only currently available MDIO bus.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>