net: dsa: felix: remove prevalidate_phy_mode interface

All users of the felix driver were creating their own prevalidate_phy_mode
function. The same logic can be performed in a more general way by using a
simple array of bit fields.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>