phy: warn if phylib and ethtool PHY mode definitions are out of sync

If new PHY modes are added people may miss to update all relevant places
in the kernel. Therefore add a build bug check for new modes in enum
ethtool_link_mode_bit_indices that haven't been added to phylib yet.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>