net: mdiobus: add APIs for modifying a MDIO device register

Add APIs for modifying a MDIO device register, similar to the existing
phy_modify() group of functions, but at mdiobus level instead.  Adapt
__phy_modify_changed() to use the new mdiobus level helper.

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