net: ag71xx: use phylink_generic_validate()

ag71xx apparently only supports MII port type, which makes it different
from other implementations. However, Oleksij says there is no special
reason for this.

Convert the driver to use phylink_generic_validate(), which will allow
all ethtool port linkmodes instead of only MII, giving the driver
consistent behaviour with other drivers.

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