net: phylink: add phylink_speed_(up|down) interface

Add an interface for the phy_speed_(up|down) functions when a driver
makes use of phylink. These pass the call through to phylib when we
have a normal PHY attached (i.o.w., not a PHY on a SFP module.)

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>