MAINTAINERS: add myself as co-maintainer for r8169

Meanwhile I know the driver quite well and I refactored bigger parts
of it. As a result people contact me already with r8169 questions.
Therefore I'd volunteer to become co-maintainer of the driver also
officially.

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