net: mii: add generic function to support ksetting support

The old ethtool api (get_setting and set_setting) has generic mii
functions mii_ethtool_sset and mii_ethtool_gset.

To support the new ethtool api ({get|set}_link_ksettings), we add
two generics mii function mii_ethtool_{get|set}_link_ksettings_get.

Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>