net: phy: add phy_speed_down_core and phy_resolve_min_speed

phy_speed_down_core provides most of the functionality for
phy_speed_down. It makes use of new helper phy_resolve_min_speed that is
based on the sorting of the settings[] array. In certain cases it may be
helpful to be able to exclude legacy half duplex modes, therefore
prepare phy_resolve_min_speed() for it.

v2:
- rename __phy_speed_down to phy_speed_down_core

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