net: dsa: lantiq: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the Lantiq
DSA switches and remove the old validate implementation to allow DSA to
use phylink_generic_validate() for this switch driver.

The exclusion of Gigabit linkmodes for MII, Reverse MII and Reduced MII
links is handled within phylink_generic_validate() in phylink, so there
is no need to make them conditional on the interface mode in the driver.

Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>