net: ag71xx: extend link validation to support other SoCs

Most (all?) QCA SoCs have two MAC with different supported link
capabilities. Extend ag71xx_mac_validate() to properly validate this
variants.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>