mdio: Move mdiobus_read/write operatings into mdio.h

These are logically MDIO operations, not phy operations, so move them
into the mdio header.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>