net: dsa: ar9331: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the AR9331
DSA switch and remove the old validate implementation to allow DSA to
use phylink_generic_validate() for this switch driver.

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