net: phy: realtek: read actual speed to detect downshift

At least some integrated PHY's in RTL8168/RTL8125 chip versions support
downshift, and the actual link speed can be read from a vendor-specific
register. Info about this register was provided by Realtek.
More details about downshift configuration (e.g. number of attempts)
aren't available, therefore the downshift tunable is not implemented.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>