net: phy: realtek: use generic MDIO constants

Drop the ad-hoc MDIO constants used in the driver and use generic
constants instead.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/732a70d6-4191-4aae-8862-3716b062aa9e@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>