net: dsa: qca8k: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the QCA8K
DSA switch and remove the old validate implementation to allow DSA to
use phylink_generic_validate() for this switch driver.

In making this change, we bring consistency to the ethtool linkmodes
that phylink's validate step produces, thereby following the expected
behaviour as the phylink documentation has explained. Specifically, the
ethtool 1000baseX_Full capability is now permitted for all interface
modes, as it is a property of the PHY driver whether 1000baseX fiber
connections can be supported.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>