net: dsa: mv88e6xxx: Rename _phy_ to _mdio_

The switch implements a generic MDIO bus, which could host more than
PHYs. It is conventional to use _mdio_ or _mii_ in the function name,
so rename them. Also postfix make the historically first read/write
function with _direct, to help distinguish it from _indirect and _ppu.

While touching these functions, remove some of the _ prefixes, which
we are deprecating.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>