net: dsa: felix: use phylink_generic_validate()

Drop the custom implementation of phylink_validate() in favor of the
generic one, which requires config->mac_capabilities to be set.

This was used up until now because of the possibility of being paired
with Aquantia PHYs with support for rate matching. The phylink framework
gained generic support for these, and knows to advertise all 10/100/1000
lower speed link modes when our SERDES protocol is 2500base-x
(fixed speed).

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>