r8169: add support for RTL8125B

Add support for RTL8125B rev.b. In my tests 2.5Gbps worked well
w/o firmware, however for a stable link at 1Gbps firmware revision
0.0.2 is needed.

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