Add comments for ethtool_cmd::phy_address and
ethtool_cmd::mdio_support, and definitions of the flags currently
used in mdio_support.

In the mdio library, assert that its own flags continue to match those
in the ethtool interface.

In the mii library, use the ethtool flag definition and stop
including <linux/mdio.h>.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>