net: phy: let phy_speed_down/up support speeds >1Gbps

So far phy_speed_down/up can be used up to 1Gbps only. Remove this
restriction by using new helper __phy_speed_down. New member adv_old
in struct phy_device is used by phy_speed_up to restore the advertised
modes before calling phy_speed_down. Don't simply advertise what is
supported because a user may have intentionally removed modes from
advertisement.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>