MAINTAINERS: add Yanjun to FORCEDETH maintainers list

Yanjun has been spending quite a lot of time fixing bugs
in FORCEDETH source code. I'd like to add Yanjun to maintainers
list.

Signed-off-by: Rain River <rain.1986.08.12@gmail.com>
Acked-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>