net: mdio: add mdiobus_modify_changed()

Add mdiobus_modify_changed() helper to reflect the phylib and similar
equivalents. This will avoid this functionality being open-coded, as
has already happened in phylink, and it looks like other users will be
appearing soon.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>