r8169: change type of member mac_version in rtl8169_private

Use the appropriate enum type for member mac_version. And don't assign
a fixed value to RTL_GIGA_MAC_NONE, there's no benefit in it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>