net: stmmac: use phylink_limit_mac_speed()

Use phylink_limit_mac_speed() to limit the MAC capabilities rather
than coding this for each speed.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZAXO-005pU7-61@rmk-PC.armlinux.org.uk


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