net: mdio: add clause 73 to ethtool conversion helper

Add a helper to convert a clause 73 advertisement to an ethtool bitmap.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>