net: mdiobus: add clause 45 mdiobus accessors

There is a recurring pattern throughout some of the PHY code converting
a devad and regnum to our packed clause 45 representation. Rather than
having this scattered around the code, let's put a common translation
function in mdio.h, and provide some register accessors.

Convert the phylib core, phylink, bcm87xx and cortina to use these.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>