net: dsa: mv88e6xxx: Abstract mv88e6165 PHY operations

The mv88e6165 family has the internal PHYs mapped directly onto the
SMI register space as the switch. So the registers can be read
directly. Put a wrapper around this, in preparation for changing the
signature in order to support the external MDIO bus of the 6390.

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>