net: dsa: remove phylink_validate() method

As of now, no DSA driver uses a custom link mode validation procedure
anymore. So remove this DSA operation and let phylink determine what is
supported based on config->mac_capabilities (if provided by the driver).
Leave a comment why we left the code that we did, and that there is more
work to do.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>