net: phylink: add phylink_limit_mac_speed()

Add a function which can be used to limit the phylink MAC capabilities
to an upper speed limit.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZAX3-005pTi-K1@rmk-PC.armlinux.org.uk


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