MAINTAINERS: update FORCEDETH MAINTAINERS info

Many FORCEDETH NICs are used in our hosts. Several bugs are fixed and
some features are developed for FORCEDETH NICs. And I have been
reviewing patches for FORCEDETH NIC for several months. Mark me as the
FORCEDETH NIC maintainer. I will send out the patches and maintain
FORCEDETH NIC.

Signed-off-by: Rain River <rain.1986.08.12@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>