ethtool/mdio: Report MDIO mode support and link partner advertising

Add mdio_support and lp_advertising fields to ethtool_cmd.  Set these
in mdio45_ethtool_gset{,_npage}().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>