r8169: restrict rtl_is_8168evl_up to RTL8168 chip versions

Extend helper rtl_is_8168evl_up to properly work once we add
mac version numbers >51 for RTL8125.

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